diff --git a/src/Cargo.lock b/src/Cargo.lock
index 3efc8866..1cafb680 100644
--- a/src/Cargo.lock
+++ b/src/Cargo.lock
@@ -1846,6 +1846,7 @@ dependencies = [
 name = "obikphylo"
 version = "0.1.0"
 dependencies = [
+ "memmap2",
  "ndarray",
  "obicompactvec",
  "obikindex",
diff --git a/src/obicompactvec/src/lib.rs b/src/obicompactvec/src/lib.rs
index 1f85a49e..9041ab78 100644
--- a/src/obicompactvec/src/lib.rs
+++ b/src/obicompactvec/src/lib.rs
@@ -7,7 +7,6 @@ mod intmatrix;
 mod layer_meta;
 mod meta;
 mod reader;
-mod siblingannex;
 mod tempbitvec;
 mod tempintvec;
 mod views;
@@ -19,7 +18,6 @@ pub use builder::PersistentCompactIntVecBuilder;
 pub use colgroup::{ColGroup, FilterMask, MatrixGroupOps, eval_filter_mask};
 pub use intmatrix::{PersistentCompactIntMatrix, PersistentCompactIntMatrixBuilder, pack_compact_int_matrix};
 pub use layer_meta::LayerMeta;
-pub use siblingannex::{FamilyMask, SiblingAnnex, SiblingAnnexBuilder};
 pub use reader::{PersistentCompactIntVec, Iter as CompactIntVecIter};
 pub use tempbitvec::{TempBitVec, TempBitVecBuilder};
 pub use tempintvec::{TempCompactIntVec, TempCompactIntVecBuilder};
diff --git a/src/obicompactvec/src/siblingannex.rs b/src/obicompactvec/src/siblingannex.rs
deleted file mode 100644
index 80e72d56..00000000
--- a/src/obicompactvec/src/siblingannex.rs
+++ /dev/null
@@ -1,280 +0,0 @@
-//! Family presence-mask annex: a compact, read-only-after-build, per-slot
-//! derived value used by the central-position SNP distance estimator (see
-//! `docmd/theory/evolutionary_distances.md`, "Step 2b" and "Definitions:
-//! family, and the canonical form of a family").
-//!
-//! One byte is stored per MPHF slot of a partition/layer, its low 4 bits
-//! encoding a **presence mask** for the slot's k-mer's "family" (the up to 4
-//! k-mers sharing the same flanks, differing only at the central base):
-//! bit `b` (`b` = 0..3, in the fixed A/C/G/T = 0/1/2/3 encoding already used
-//! for a single nucleotide) is set iff the family member whose *own* central
-//! base — in its own canonical orientation — is `b`, is observed anywhere in
-//! the current multi-genome index. This is a property of the whole index,
-//! not of any one genome.
-//!
-//! Both facts the earlier (superseded) 3-bit design stored explicitly are
-//! derived from the mask instead, not stored:
-//! - sibling count = `popcount(mask) - 1`;
-//! - minorant = regenerate the family's 4 canonical forms from the slot's
-//!   own k-mer (`CanonicalKmerOf::central_canonical_neighbors`, cheap, no
-//!   lookup), compare the raw encodings of whichever are set in the mask,
-//!   take the smallest — see `obikindex::siblings`.
-//!
-//! Mask value 0 is logically unreachable as a real result (a slot's own base
-//! is always present in its own family) and is reused as the "not yet
-//! computed" sentinel: annex files are pre-initialised to all-zero, and a
-//! real value is only ever written once, by the computation pass.
-//!
-//! Deliberately simpler than a true 4-bit pack (1 byte/slot instead of 4
-//! bits/slot): correctness and simplicity first, for a first implementation.
-//! Packing to 4 bits/slot is a pure storage-density follow-up, not a
-//! behavioural change, left for later.
-
-use std::fs::{File, OpenOptions};
-use std::io;
-use std::path::{Path, PathBuf};
-
-use memmap2::{Mmap, MmapMut};
-
-const MAGIC: [u8; 4] = *b"PSIB";
-
-// Header: magic(4) + _pad(4) + n(8) = 16 bytes. Data (1 byte/slot) follows.
-const HEADER_SIZE: usize = 16;
-
-/// A family presence mask: bit `b` (0..3) set iff the member whose own
-/// canonical central base is `b` (0=A, 1=C, 2=G, 3=T) is observed in the
-/// index. Bit 4 is a second, independent fact piggy-backed onto the same
-/// byte: whether *this slot's own k-mer* is its family's minorant (the
-/// smallest raw encoding among the family's observed members) — computed
-/// once, when the whole family's mask is already final (see
-/// `obikindex::siblings::build_sibling_annex`), and read back by every
-/// consumer that would otherwise have to reconstruct this slot's k-mer from
-/// `unitigs.bin` and hash it through the MPHF again just to ask the same
-/// question. Bits 5-7 unused.
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
-pub struct FamilyMask(u8);
-
-const PRESENCE_BITS: u8 = 0b0000_1111;
-const MINORANT_BIT: u8 = 0b0001_0000;
-
-impl FamilyMask {
-    /// The empty mask — never a valid *computed* result (a slot's own base
-    /// is always present in its own family) — used only to build up a mask
-    /// via repeated [`with`](Self::with) calls before storing it.
-    pub const EMPTY: FamilyMask = FamilyMask(0);
-
-    /// Set bit `base` (0=A, 1=C, 2=G, 3=T).
-    #[inline]
-    pub fn with(self, base: u8) -> Self {
-        debug_assert!(base < 4, "base out of range: {base}");
-        FamilyMask(self.0 | (1 << base))
-    }
-
-    /// Is the member with central base `base` (0..3) present?
-    #[inline]
-    pub fn has(self, base: u8) -> bool {
-        debug_assert!(base < 4, "base out of range: {base}");
-        self.0 & (1 << base) != 0
-    }
-
-    /// Number of family members observed anywhere in the index (1..=4).
-    #[inline]
-    pub fn family_size(self) -> u32 {
-        (self.0 & PRESENCE_BITS).count_ones()
-    }
-
-    /// Number of *other* members observed (0..=3) — `family_size() - 1`.
-    #[inline]
-    pub fn siblings(self) -> u32 {
-        self.family_size() - 1
-    }
-
-    /// Set or clear the minorant flag (see the struct docs).
-    #[inline]
-    pub fn with_minorant(self, is_minorant: bool) -> Self {
-        if is_minorant {
-            FamilyMask(self.0 | MINORANT_BIT)
-        } else {
-            FamilyMask(self.0 & !MINORANT_BIT)
-        }
-    }
-
-    /// Is this slot's own k-mer its family's minorant? Only meaningful once
-    /// `with_minorant` has been called with the family's *final* mask (i.e.
-    /// after construction) — see the struct docs.
-    #[inline]
-    pub fn is_minorant(self) -> bool {
-        self.0 & MINORANT_BIT != 0
-    }
-
-    /// Raw presence bitmask (bit `b` = base `b` present, low 4 bits only —
-    /// never includes the minorant flag) — for callers that build up a mask
-    /// via their own bit operations (e.g. concurrently, via an `AtomicU8`)
-    /// and only need the `FamilyMask` wrapper at the end.
-    #[inline]
-    pub fn bits(self) -> u8 {
-        self.0 & PRESENCE_BITS
-    }
-
-    /// Construct from a raw presence bitmask (only the low 4 bits are kept —
-    /// the minorant flag is not part of this, use [`with_minorant`](Self::with_minorant)
-    /// separately).
-    #[inline]
-    pub fn from_bits(bits: u8) -> Self {
-        FamilyMask(bits & PRESENCE_BITS)
-    }
-
-    #[inline]
-    fn encode(self) -> u8 {
-        self.0
-    }
-
-    #[inline]
-    fn decode(byte: u8) -> Option<Self> {
-        if byte == 0 {
-            // Unreachable for a real result — reserved as the "not yet
-            // computed" sentinel. Still safe as a sentinel with the
-            // minorant bit added: a real entry always has at least one
-            // presence bit set (bits 0-3), so byte == 0 still means
-            // "nothing written yet" and never a genuine minorant-only value.
-            return None;
-        }
-        Some(FamilyMask(byte))
-    }
-}
-
-// ── SiblingAnnex (reader) ───────────────────────────────────────────────────
-
-pub struct SiblingAnnex {
-    mmap: Mmap,
-    n: usize,
-    path: PathBuf,
-}
-
-impl SiblingAnnex {
-    pub fn open(path: &Path) -> io::Result<Self> {
-        let mmap = unsafe { Mmap::map(&File::open(path)?)? };
-        if mmap.len() < HEADER_SIZE {
-            return Err(io::Error::new(io::ErrorKind::InvalidData, "PSIB file too short"));
-        }
-        if mmap[0..4] != MAGIC {
-            return Err(io::Error::new(io::ErrorKind::InvalidData, "bad PSIB magic"));
-        }
-        let n = u64::from_le_bytes(mmap[8..16].try_into().unwrap()) as usize;
-        if mmap.len() < HEADER_SIZE + n {
-            return Err(io::Error::new(io::ErrorKind::InvalidData, "PSIB file truncated"));
-        }
-        Ok(Self { mmap, n, path: path.to_path_buf() })
-    }
-
-    pub fn path(&self) -> &Path { &self.path }
-    pub fn len(&self) -> usize { self.n }
-    pub fn is_empty(&self) -> bool { self.n == 0 }
-
-    /// `None` means the slot has not (yet) been computed — see module docs.
-    pub fn get(&self, slot: usize) -> Option<FamilyMask> {
-        FamilyMask::decode(self.mmap[HEADER_SIZE + slot])
-    }
-}
-
-// ── SiblingAnnexBuilder (writer) ────────────────────────────────────────────
-
-pub struct SiblingAnnexBuilder {
-    mmap: MmapMut,
-    n: usize,
-    path: PathBuf,
-}
-
-impl SiblingAnnexBuilder {
-    /// Create a new annex of `n` slots at `path`, pre-initialised to the
-    /// "not yet computed" sentinel (all-zero).
-    pub fn new(n: usize, path: &Path) -> io::Result<Self> {
-        let file_size = HEADER_SIZE + n;
-        let file = OpenOptions::new()
-            .read(true).write(true).create(true).truncate(true)
-            .open(path)?;
-        file.set_len(file_size as u64)?;
-        let mut mmap = unsafe { MmapMut::map_mut(&file)? };
-        mmap[0..4].copy_from_slice(&MAGIC);
-        mmap[4..8].copy_from_slice(&[0u8; 4]);
-        mmap[8..16].copy_from_slice(&(n as u64).to_le_bytes());
-        // Data region left at 0 by `set_len`/mmap — the sentinel value.
-        Ok(Self { mmap, n, path: path.to_path_buf() })
-    }
-
-    pub fn len(&self) -> usize { self.n }
-    pub fn is_empty(&self) -> bool { self.n == 0 }
-
-    pub fn get(&self, slot: usize) -> Option<FamilyMask> {
-        FamilyMask::decode(self.mmap[HEADER_SIZE + slot])
-    }
-
-    pub fn set(&mut self, slot: usize, mask: FamilyMask) {
-        // Redundant concurrent writes from independent recomputation paths
-        // converge to the same encoded byte for a given slot, so a plain
-        // store here is safe even without external synchronisation, as long
-        // as the byte write itself is atomic (true for a single aligned
-        // byte on every platform this project targets).
-        self.mmap[HEADER_SIZE + slot] = mask.encode();
-    }
-
-    pub fn close(self) -> io::Result<()> { self.mmap.flush() }
-
-    pub fn finish(self) -> io::Result<SiblingAnnex> {
-        let path = self.path.clone();
-        self.close()?;
-        SiblingAnnex::open(&path)
-    }
-}
-
-#[cfg(test)]
-mod tests {
-    use super::*;
-    use tempfile::tempdir;
-
-    #[test]
-    fn sentinel_is_zero_and_unset_slots_read_as_uncomputed() {
-        let dir = tempdir().unwrap();
-        let path = dir.path().join("test.psib");
-        let builder = SiblingAnnexBuilder::new(4, &path).unwrap();
-        for slot in 0..4 {
-            assert_eq!(builder.get(slot), None);
-        }
-        builder.close().unwrap();
-    }
-
-    #[test]
-    fn roundtrip_all_valid_masks() {
-        let dir = tempdir().unwrap();
-        let path = dir.path().join("test.psib");
-        let mut builder = SiblingAnnexBuilder::new(4, &path).unwrap();
-
-        let masks = [
-            FamilyMask::EMPTY.with(0),                   // just A: family size 1
-            FamilyMask::EMPTY.with(0).with(3),            // A + T: size 2
-            FamilyMask::EMPTY.with(1).with(2).with(3),    // C+G+T: size 3
-            FamilyMask::EMPTY.with(0).with(1).with(2).with(3), // all 4
-        ];
-        for (slot, mask) in masks.iter().enumerate() {
-            builder.set(slot, *mask);
-        }
-        let annex = builder.finish().unwrap();
-        for (slot, mask) in masks.iter().enumerate() {
-            assert_eq!(annex.get(slot), Some(*mask));
-        }
-        assert_eq!(annex.get(0).unwrap().siblings(), 0);
-        assert_eq!(annex.get(1).unwrap().siblings(), 1);
-        assert_eq!(annex.get(2).unwrap().siblings(), 2);
-        assert_eq!(annex.get(3).unwrap().siblings(), 3);
-        assert_eq!(annex.get(3).unwrap().family_size(), 4);
-    }
-
-    #[test]
-    fn has_reflects_individual_bits() {
-        let mask = FamilyMask::EMPTY.with(0).with(2);
-        assert!(mask.has(0));
-        assert!(!mask.has(1));
-        assert!(mask.has(2));
-        assert!(!mask.has(3));
-    }
-}
diff --git a/src/obikphylo/Cargo.toml b/src/obikphylo/Cargo.toml
index be66a13c..5dc257ba 100644
--- a/src/obikphylo/Cargo.toml
+++ b/src/obikphylo/Cargo.toml
@@ -13,6 +13,7 @@ obicompactvec    = { path = "../obicompactvec" }
 obilayeredmap    = { path = "../obilayeredmap" }
 obiskbuilder     = { path = "../obiskbuilder" }
 obipipeline      = { path = "../obipipeline" }
+memmap2          = "0.9"
 ndarray          = "0.16"
 rayon            = "1"
 tracing          = "0.1.44"
diff --git a/src/obikphylo/src/siblings/build.rs b/src/obikphylo/src/siblings/build.rs
index b81d160b..5a541e5f 100644
--- a/src/obikphylo/src/siblings/build.rs
+++ b/src/obikphylo/src/siblings/build.rs
@@ -4,7 +4,6 @@ use std::sync::Arc;
 
 use rayon::prelude::*;
 
-use obicompactvec::{FamilyMask, SiblingAnnexBuilder};
 use obikpartitionner::KmerPartition;
 use obipipeline::ThrottleGuard;
 use obikseq::CanonicalKmer;
@@ -17,7 +16,7 @@ use obikindex::KmerIndex;
 
 use super::cache::PartitionCache;
 use super::helpers::{central_base, is_minorant};
-use super::{olm_to_ok, ANNEX_FILE_NAME, INDEX_SUBDIR};
+use super::{olm_to_ok, FamilyMask, SiblingAnnexBuilder, ANNEX_FILE_NAME, INDEX_SUBDIR};
 
 // ── obipipeline data types ─────────────────────────────────────────────────
 
diff --git a/src/obikphylo/src/siblings/cache.rs b/src/obikphylo/src/siblings/cache.rs
index 8d706d27..e3047821 100644
--- a/src/obikphylo/src/siblings/cache.rs
+++ b/src/obikphylo/src/siblings/cache.rs
@@ -10,7 +10,7 @@ use obisys::progress_bar;
 use obikindex::OKIResult;
 
 use super::iter::SiblingLayerExt;
-use super::{olm_to_ok, INDEX_SUBDIR};
+use super::{olm_to_ok, SiblingAnnex, INDEX_SUBDIR};
 
 /// Every partition's already-open layers, built **once** for the whole
 /// `build_sibling_annex` run and shared (read-only) across every lookup, in
@@ -68,7 +68,7 @@ impl Mat {
     /// doesn't depend on `D`), so no boxing is needed.
     pub(super) fn iter_minorants_batch(
         &self,
-        annex: std::sync::Arc<obicompactvec::SiblingAnnex>,
+        annex: std::sync::Arc<SiblingAnnex>,
         batch_size: usize,
     ) -> super::iter::MinorantBatchIter {
         match self {
diff --git a/src/obikphylo/src/siblings/family_scan.rs b/src/obikphylo/src/siblings/family_scan.rs
index 56bc5d4c..fdde2249 100644
--- a/src/obikphylo/src/siblings/family_scan.rs
+++ b/src/obikphylo/src/siblings/family_scan.rs
@@ -53,7 +53,7 @@ use std::sync::atomic::{AtomicU8, Ordering};
 
 use rayon::prelude::*;
 
-use obicompactvec::{FamilyMask, PersistentBitMatrix, PersistentCompactIntMatrix, SiblingAnnex};
+use obicompactvec::{PersistentBitMatrix, PersistentCompactIntMatrix};
 use obikseq::CanonicalKmer;
 use obilayeredmap::Layer;
 use obilayeredmap::meta::PartitionMeta;
@@ -65,7 +65,7 @@ use obikindex::KmerIndex;
 use super::cache::{Mat, PartitionCache};
 use super::helpers::central_base;
 use super::iter::SiblingEntry;
-use super::{olm_to_ok, ANNEX_FILE_NAME, INDEX_SUBDIR};
+use super::{olm_to_ok, FamilyMask, SiblingAnnex, ANNEX_FILE_NAME, INDEX_SUBDIR};
 
 /// Families per batch — see the module docs for the memory-vs-per-partition-
 /// density trade-off this picks a point on. At ~90 genomes and a few
diff --git a/src/obikphylo/src/siblings/helpers.rs b/src/obikphylo/src/siblings/helpers.rs
index 580162b4..b11c7c52 100644
--- a/src/obikphylo/src/siblings/helpers.rs
+++ b/src/obikphylo/src/siblings/helpers.rs
@@ -1,6 +1,6 @@
 use obikseq::CanonicalKmer;
 
-use obicompactvec::FamilyMask;
+use super::FamilyMask;
 
 /// Central-position base of a canonical k-mer, in the fixed 0=A/1=C/2=G/3=T
 /// encoding — the mask's bit index. `k` must be odd (project invariant).
diff --git a/src/obikphylo/src/siblings/iter.rs b/src/obikphylo/src/siblings/iter.rs
index 20bea078..932f0eed 100644
--- a/src/obikphylo/src/siblings/iter.rs
+++ b/src/obikphylo/src/siblings/iter.rs
@@ -28,10 +28,11 @@
 
 use std::sync::Arc;
 
-use obicompactvec::{FamilyMask, SiblingAnnex};
 use obikseq::CanonicalKmer;
 use obilayeredmap::{KmerIter, Layer, LayerData};
 
+use super::{FamilyMask, SiblingAnnex};
+
 /// One layer entry: a k-mer's position in the layer's iteration order (the
 /// same index the sibling annex is keyed on — not an MPHF slot), the k-mer
 /// itself, and its family mask.
diff --git a/src/obikphylo/src/siblings/mod.rs b/src/obikphylo/src/siblings/mod.rs
index cd3f5da8..9fa04f5e 100644
--- a/src/obikphylo/src/siblings/mod.rs
+++ b/src/obikphylo/src/siblings/mod.rs
@@ -50,6 +50,7 @@ mod distance;
 mod family_scan;
 mod helpers;
 mod iter;
+mod siblingannex;
 mod stats;
 
 #[cfg(test)]
@@ -60,6 +61,7 @@ pub use build::SiblingAnnexBuildExt;
 pub use cardinality::{CardinalityExt, CardinalityTally};
 pub use distance::{BasePairTally, DistanceExt, RawSnpDistanceOutput};
 pub use iter::{MinorantBatchIter, MinorantIter, SiblingBatchIter, SiblingEntry, SiblingIter, SiblingLayerExt};
+pub(crate) use siblingannex::{FamilyMask, SiblingAnnex, SiblingAnnexBuilder};
 pub use stats::{SiblingAnnexStats, SiblingStatsExt};
 
 use obilayeredmap::OLMError;
diff --git a/src/obikphylo/src/siblings/siblingannex.rs b/src/obikphylo/src/siblings/siblingannex.rs
new file mode 100644
index 00000000..38f62aee
--- /dev/null
+++ b/src/obikphylo/src/siblings/siblingannex.rs
@@ -0,0 +1,390 @@
+//! Family presence-mask annex: a compact, read-only-after-build, per-slot
+//! derived value used by the central-position SNP distance estimator (see
+//! `docmd/theory/evolutionary_distances.md`, "Step 2b" and "Definitions:
+//! family, and the canonical form of a family").
+//!
+//! Two bytes are stored per MPHF slot of a partition/layer, packed as four
+//! 3-bit fields (one per central base, in the fixed A/C/G/T = 0/1/2/3
+//! encoding) plus a minorant flag:
+//!
+//! - Each 3-bit field: `0` = the family member whose *own* central base —
+//!   in its own canonical orientation — is that base is absent from the
+//!   whole multi-genome index (a property of the whole index, not of any
+//!   one genome); a non-zero value `v` (`1..=7`) means it's present, and
+//!   *may* additionally encode `layer = v - 1` (the destination layer
+//!   within its partition) once a caller populates it via
+//!   [`with_layer`](FamilyMask::with_layer) — see that method's docs.
+//!   `FamilyMask` itself is policy-free about whether a given non-zero
+//!   value is a trustworthy layer index or just "present, no layer
+//!   recorded": that decision belongs to callers, who already have
+//!   `PartitionMeta::n_layers` in scope.
+//! - The minorant bit: whether *this slot's own k-mer* is its family's
+//!   minorant (the smallest raw encoding among the family's observed
+//!   members) — computed once, when the whole family's mask is already
+//!   final (see `build::build_layer_sibling_annex`), and read back by every
+//!   consumer that would otherwise have to reconstruct this slot's k-mer
+//!   from `unitigs.bin` and hash it through the MPHF again just to ask the
+//!   same question.
+//!
+//! Both facts an even earlier design stored explicitly are still derived,
+//! not stored:
+//! - sibling count = `family_size() - 1`;
+//! - which of the (up to 4) family members are present, and their canonical
+//!   form — [`FamilyMask::family_members`], regenerated from the slot's own
+//!   k-mer (`CanonicalKmer::central_canonical_neighbors`, cheap, no
+//!   lookup), not stored.
+//!
+//! Mask value 0 is logically unreachable as a real result (a slot's own
+//! base is always present in its own family) and is reused as the "not yet
+//! computed" sentinel: annex files are pre-initialised to all-zero, and a
+//! real value is only ever written once, by the computation pass.
+//!
+//! Widened from the original 1-byte/slot design (4 presence bits + 1
+//! minorant bit) to carry a per-member layer number without an extra
+//! lookup pass — see the project discussion this implements. An old
+//! (1-byte/slot) `.psib` file is *not* silently misread by the new reader:
+//! its length no longer matches `HEADER_SIZE + n * 2`, so `SiblingAnnex::open`
+//! fails loudly ("PSIB file truncated") rather than producing garbage.
+//! Annexes built before this change must be rebuilt (`--sibling-annex`).
+
+use std::fs::{File, OpenOptions};
+use std::io;
+use std::path::{Path, PathBuf};
+
+use memmap2::{Mmap, MmapMut};
+
+use obikseq::CanonicalKmer;
+
+use super::helpers::central_base;
+
+const MAGIC: [u8; 4] = *b"PSIB";
+
+// Header: magic(4) + _pad(4) + n(8) = 16 bytes. Data (2 bytes/slot) follows.
+const HEADER_SIZE: usize = 16;
+
+/// Width, in bits, of one base's field.
+const FIELD_BITS: u32 = 3;
+/// Mask for one base's field once shifted into position.
+const FIELD_MASK: u16 = 0b111;
+/// Highest raw field value a base can carry (`1..=MAX_FIELD_VALUE`, `0` = absent).
+const MAX_FIELD_VALUE: u8 = 7;
+
+const MINORANT_BIT: u16 = 1 << 12;
+
+/// A family presence mask — see the module docs for the full bit layout.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct FamilyMask(u16);
+
+impl FamilyMask {
+    /// The empty mask — never a valid *computed* result (a slot's own base
+    /// is always present in its own family) — used only to build up a mask
+    /// via repeated [`with`](Self::with)/[`with_layer`](Self::with_layer)
+    /// calls before storing it.
+    pub const EMPTY: FamilyMask = FamilyMask(0);
+
+    #[inline]
+    fn field_shift(base: u8) -> u32 {
+        debug_assert!(base < 4, "base out of range: {base}");
+        base as u32 * FIELD_BITS
+    }
+
+    #[inline]
+    fn field(self, base: u8) -> u8 {
+        ((self.0 >> Self::field_shift(base)) & FIELD_MASK) as u8
+    }
+
+    #[inline]
+    fn with_field(self, base: u8, value: u8) -> Self {
+        debug_assert!(value <= MAX_FIELD_VALUE, "field value out of range: {value}");
+        let shift = Self::field_shift(base);
+        let cleared = self.0 & !(FIELD_MASK << shift);
+        FamilyMask(cleared | ((value as u16) << shift))
+    }
+
+    /// Mark the member with central base `base` (0=A, 1=C, 2=G, 3=T) as
+    /// present, without recording a layer (compat path: same observable
+    /// effect as the original 1-byte design's `with`). Use
+    /// [`with_layer`](Self::with_layer) instead when the destination layer
+    /// is already known.
+    #[inline]
+    pub fn with(self, base: u8) -> Self {
+        self.with_field(base, 1)
+    }
+
+    /// Mark the member with central base `base` as present *and* record its
+    /// destination layer. `layer` must be `< 7` (`debug_assert`ed) — a
+    /// caller facing more layers than that has no compact field to record
+    /// them in and must fall back to [`with`](Self::with) instead (still
+    /// correct, just without the fast-path payoff); see the module docs on
+    /// why `FamilyMask` doesn't decide that threshold itself.
+    #[inline]
+    pub fn with_layer(self, base: u8, layer: usize) -> Self {
+        debug_assert!(layer < MAX_FIELD_VALUE as usize, "layer out of range: {layer}");
+        self.with_field(base, layer as u8 + 1)
+    }
+
+    /// Is the member with central base `base` (0..3) present?
+    #[inline]
+    pub fn has(self, base: u8) -> bool {
+        self.field(base) != 0
+    }
+
+    /// Raw stored value for `base`'s field, if present: `Some(v - 1)` where
+    /// `v` is the non-zero field value. This is the *raw* stored value, not
+    /// a validated layer index — a caller must cross-check it against
+    /// `PartitionMeta::n_layers` (`<= 7`) before trusting it as a real
+    /// layer, since a mask written via [`with`](Self::with) (no layer
+    /// known) also reads back as `Some(0)` here.
+    #[inline]
+    pub fn layer_value(self, base: u8) -> Option<u8> {
+        let v = self.field(base);
+        if v == 0 { None } else { Some(v - 1) }
+    }
+
+    /// Number of family members observed anywhere in the index (1..=4).
+    #[inline]
+    pub fn family_size(self) -> u32 {
+        (0..4).filter(|&b| self.has(b)).count() as u32
+    }
+
+    /// Number of *other* members observed (0..=3) — `family_size() - 1`.
+    #[inline]
+    pub fn siblings(self) -> u32 {
+        self.family_size() - 1
+    }
+
+    /// Set or clear the minorant flag (see the module docs).
+    #[inline]
+    pub fn with_minorant(self, is_minorant: bool) -> Self {
+        if is_minorant {
+            FamilyMask(self.0 | MINORANT_BIT)
+        } else {
+            FamilyMask(self.0 & !MINORANT_BIT)
+        }
+    }
+
+    /// Is this slot's own k-mer its family's minorant? Only meaningful once
+    /// `with_minorant` has been called with the family's *final* mask (i.e.
+    /// after construction) — see the module docs.
+    #[inline]
+    pub fn is_minorant(self) -> bool {
+        self.0 & MINORANT_BIT != 0
+    }
+
+    /// Raw presence bitmask (bit `b` = base `b` present, low 4 bits only —
+    /// never includes the minorant flag or any layer information) — for
+    /// callers that only need the old 1-bit-per-base view, e.g. to compare
+    /// against a previously-computed value built via their own bit
+    /// operations.
+    #[inline]
+    pub fn bits(self) -> u8 {
+        (0..4).fold(0u8, |acc, b| if self.has(b) { acc | (1 << b) } else { acc })
+    }
+
+    /// Construct from a raw presence bitmask (only the low 4 bits are kept
+    /// — the minorant flag is not part of this, use
+    /// [`with_minorant`](Self::with_minorant) separately). Each set bit is
+    /// recorded as "present, no layer known" — see [`with`](Self::with).
+    #[inline]
+    pub fn from_bits(bits: u8) -> Self {
+        (0..4).fold(FamilyMask::EMPTY, |mask, b| {
+            if bits & (1 << b) != 0 { mask.with(b) } else { mask }
+        })
+    }
+
+    /// This family's present members, in canonical form, paired with their
+    /// raw stored field value (see [`layer_value`](Self::layer_value) for
+    /// why it's not directly a validated layer index). `owner` is this
+    /// slot's own k-mer (any member works — the 4 canonical forms are
+    /// invariant regardless of which member you start from).
+    pub fn family_members(self, owner: CanonicalKmer, k: usize) -> impl Iterator<Item = (CanonicalKmer, Option<u8>)> {
+        owner
+            .central_canonical_neighbors()
+            .into_iter()
+            .filter_map(move |member| {
+                let base = central_base(member, k);
+                self.has(base).then(|| (member, self.layer_value(base)))
+            })
+    }
+
+    #[inline]
+    fn encode(self) -> u16 {
+        self.0
+    }
+
+    #[inline]
+    fn decode(word: u16) -> Option<Self> {
+        if word == 0 {
+            // Unreachable for a real result — reserved as the "not yet
+            // computed" sentinel. A real entry always has at least one
+            // non-zero base field, so word == 0 always means "nothing
+            // written yet", never a genuine minorant-only value.
+            return None;
+        }
+        Some(FamilyMask(word))
+    }
+}
+
+// ── SiblingAnnex (reader) ───────────────────────────────────────────────────
+
+pub struct SiblingAnnex {
+    mmap: Mmap,
+    n: usize,
+    path: PathBuf,
+}
+
+impl SiblingAnnex {
+    pub fn open(path: &Path) -> io::Result<Self> {
+        let mmap = unsafe { Mmap::map(&File::open(path)?)? };
+        if mmap.len() < HEADER_SIZE {
+            return Err(io::Error::new(io::ErrorKind::InvalidData, "PSIB file too short"));
+        }
+        if mmap[0..4] != MAGIC {
+            return Err(io::Error::new(io::ErrorKind::InvalidData, "bad PSIB magic"));
+        }
+        let n = u64::from_le_bytes(mmap[8..16].try_into().unwrap()) as usize;
+        if mmap.len() < HEADER_SIZE + n * 2 {
+            return Err(io::Error::new(io::ErrorKind::InvalidData, "PSIB file truncated"));
+        }
+        Ok(Self { mmap, n, path: path.to_path_buf() })
+    }
+
+    pub fn path(&self) -> &Path { &self.path }
+    pub fn len(&self) -> usize { self.n }
+    pub fn is_empty(&self) -> bool { self.n == 0 }
+
+    /// `None` means the slot has not (yet) been computed — see module docs.
+    pub fn get(&self, slot: usize) -> Option<FamilyMask> {
+        let off = HEADER_SIZE + slot * 2;
+        FamilyMask::decode(u16::from_le_bytes(self.mmap[off..off + 2].try_into().unwrap()))
+    }
+}
+
+// ── SiblingAnnexBuilder (writer) ────────────────────────────────────────────
+
+pub struct SiblingAnnexBuilder {
+    mmap: MmapMut,
+}
+
+impl SiblingAnnexBuilder {
+    /// Create a new annex of `n` slots at `path`, pre-initialised to the
+    /// "not yet computed" sentinel (all-zero).
+    pub fn new(n: usize, path: &Path) -> io::Result<Self> {
+        let file_size = HEADER_SIZE + n * 2;
+        let file = OpenOptions::new()
+            .read(true).write(true).create(true).truncate(true)
+            .open(path)?;
+        file.set_len(file_size as u64)?;
+        let mut mmap = unsafe { MmapMut::map_mut(&file)? };
+        mmap[0..4].copy_from_slice(&MAGIC);
+        mmap[4..8].copy_from_slice(&[0u8; 4]);
+        mmap[8..16].copy_from_slice(&(n as u64).to_le_bytes());
+        // Data region left at 0 by `set_len`/mmap — the sentinel value.
+        Ok(Self { mmap })
+    }
+
+    pub fn set(&mut self, slot: usize, mask: FamilyMask) {
+        // Redundant concurrent writes from independent recomputation paths
+        // converge to the same encoded value for a given slot, so a plain
+        // store here is safe even without external synchronisation, as long
+        // as the write itself doesn't tear — true for a 2-byte-aligned
+        // `u16` store on every platform this project targets.
+        let off = HEADER_SIZE + slot * 2;
+        self.mmap[off..off + 2].copy_from_slice(&mask.encode().to_le_bytes());
+    }
+
+    pub fn close(self) -> io::Result<()> { self.mmap.flush() }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use tempfile::tempdir;
+
+    #[test]
+    fn sentinel_is_zero_and_unset_slots_read_as_uncomputed() {
+        let dir = tempdir().unwrap();
+        let path = dir.path().join("test.psib");
+        let builder = SiblingAnnexBuilder::new(4, &path).unwrap();
+        builder.close().unwrap();
+        let annex = SiblingAnnex::open(&path).unwrap();
+        for slot in 0..4 {
+            assert_eq!(annex.get(slot), None);
+        }
+    }
+
+    #[test]
+    fn roundtrip_all_valid_masks() {
+        let dir = tempdir().unwrap();
+        let path = dir.path().join("test.psib");
+        let mut builder = SiblingAnnexBuilder::new(4, &path).unwrap();
+
+        let masks = [
+            FamilyMask::EMPTY.with(0),                   // just A: family size 1
+            FamilyMask::EMPTY.with(0).with(3),            // A + T: size 2
+            FamilyMask::EMPTY.with(1).with(2).with(3),    // C+G+T: size 3
+            FamilyMask::EMPTY.with(0).with(1).with(2).with(3), // all 4
+        ];
+        for (slot, mask) in masks.iter().enumerate() {
+            builder.set(slot, *mask);
+        }
+        builder.close().unwrap();
+        let annex = SiblingAnnex::open(&path).unwrap();
+        for (slot, mask) in masks.iter().enumerate() {
+            assert_eq!(annex.get(slot), Some(*mask));
+        }
+        assert_eq!(annex.get(0).unwrap().siblings(), 0);
+        assert_eq!(annex.get(1).unwrap().siblings(), 1);
+        assert_eq!(annex.get(2).unwrap().siblings(), 2);
+        assert_eq!(annex.get(3).unwrap().siblings(), 3);
+        assert_eq!(annex.get(3).unwrap().family_size(), 4);
+    }
+
+    #[test]
+    fn has_reflects_individual_bits() {
+        let mask = FamilyMask::EMPTY.with(0).with(2);
+        assert!(mask.has(0));
+        assert!(!mask.has(1));
+        assert!(mask.has(2));
+        assert!(!mask.has(3));
+    }
+
+    #[test]
+    fn with_layer_roundtrips_and_leaves_absent_bases_none() {
+        let mask = FamilyMask::EMPTY.with_layer(0, 0).with_layer(2, 5);
+        assert_eq!(mask.layer_value(0), Some(0));
+        assert_eq!(mask.layer_value(2), Some(5));
+        assert_eq!(mask.layer_value(1), None);
+        assert_eq!(mask.layer_value(3), None);
+        assert!(mask.has(0) && mask.has(2));
+        assert!(!mask.has(1) && !mask.has(3));
+        assert_eq!(mask.family_size(), 2);
+    }
+
+    #[test]
+    fn with_marks_present_without_a_layer() {
+        // Compat path: `with` still means "present", now reading back as
+        // `layer_value == Some(0)` (raw field value 1, i.e. "no layer
+        // recorded") — distinguishable from a real layer 0 only by a
+        // caller that already knows whether layers were ever wired in.
+        let mask = FamilyMask::EMPTY.with(1);
+        assert!(mask.has(1));
+        assert_eq!(mask.layer_value(1), Some(0));
+    }
+
+    #[test]
+    fn family_members_reconstructs_present_canonical_forms() {
+        use obikseq::{Kmer, Sequence};
+
+        const K: usize = 11;
+        obikseq::params::set_k(K);
+        // Centre (index 5) is 'C' (base 1) in this k-mer's own orientation.
+        let owner = Kmer::from_ascii(b"AACCGCTTAAG").unwrap().canonical();
+        let mask = FamilyMask::EMPTY.with_layer(1, 0).with_layer(2, 3); // C (own) + G present
+
+        let members: Vec<_> = mask.family_members(owner, K).collect();
+        assert_eq!(members.len(), 2, "only the 2 present members should be yielded");
+        assert!(members.iter().any(|(k, layer)| *k == owner && *layer == Some(0)));
+        assert!(members.iter().any(|(k, layer)| *k != owner && *layer == Some(3)));
+    }
+}
diff --git a/src/obikphylo/src/siblings/stats.rs b/src/obikphylo/src/siblings/stats.rs
index a3edd1f2..e612257f 100644
--- a/src/obikphylo/src/siblings/stats.rs
+++ b/src/obikphylo/src/siblings/stats.rs
@@ -2,7 +2,6 @@ use std::sync::Arc;
 
 use rayon::prelude::*;
 
-use obicompactvec::SiblingAnnex;
 use obikpartitionner::KmerPartition;
 use obisys::progress_bar;
 
@@ -10,6 +9,7 @@ use obikindex::{OKIError, OKIResult};
 use obikindex::KmerIndex;
 
 use super::ANNEX_FILE_NAME;
+use super::SiblingAnnex;
 use super::cache::PartitionCache;
 use super::family_scan::scan_layer_families;
 
diff --git a/src/obikphylo/src/siblings/tests.rs b/src/obikphylo/src/siblings/tests.rs
index d854bcab..702d8365 100644
--- a/src/obikphylo/src/siblings/tests.rs
+++ b/src/obikphylo/src/siblings/tests.rs
@@ -1,7 +1,6 @@
 use std::io::Write;
 use std::path::Path;
 
-use obicompactvec::{FamilyMask, SiblingAnnex};
 use obikseq::{CanonicalKmer, Kmer, Sequence};
 use obilayeredmap::MphfLayer;
 use obilayeredmap::meta::PartitionMeta;
@@ -16,7 +15,7 @@ use super::cardinality::CardinalityExt;
 use super::distance::DistanceExt;
 use super::helpers::is_minorant;
 use super::stats::SiblingStatsExt;
-use super::{ANNEX_FILE_NAME, INDEX_SUBDIR};
+use super::{FamilyMask, SiblingAnnex, ANNEX_FILE_NAME, INDEX_SUBDIR};
 
 // k must be >= 11 (project constraint, "k ∈ [11,31]"); k=11, level_max=1,
 // theta=0.0 mirror `obiskbuilder`'s own tests (smaller k/level_max