diff --git a/backend/endpoints/roms/__init__.py b/backend/endpoints/roms/__init__.py
index bb62e9b7c..99efad7ec 100644
--- a/backend/endpoints/roms/__init__.py
+++ b/backend/endpoints/roms/__init__.py
@@ -580,19 +580,26 @@ def get_roms(
 
         def _transform(items: Sequence[Rom]) -> list[SimpleRomSchema]:
             rom_ids = [i.id for i in items]
-            sibling_ids_by_rom = db_rom_handler.get_sibling_ids_for_roms(
-                rom_ids, session=session
-            )
             files_by_rom = (
                 db_rom_handler.get_files_for_roms(rom_ids, session=session)
                 if with_files
                 else {}
             )
-            siblings_by_rom = (
-                db_rom_handler.get_siblings_for_roms(rom_ids, session=session)
-                if with_siblings
-                else {}
-            )
+
+            if with_siblings:
+                siblings_by_rom = db_rom_handler.get_siblings_for_roms(
+                    rom_ids, session=session
+                )
+                # Derive the id list from the full siblings instead of a second query.
+                sibling_ids_by_rom = {
+                    rid: sorted({s.id for s in sibs})
+                    for rid, sibs in siblings_by_rom.items()
+                }
+            else:
+                siblings_by_rom = {}
+                sibling_ids_by_rom = db_rom_handler.get_sibling_ids_for_roms(
+                    rom_ids, session=session
+                )
 
             return [
                 SimpleRomSchema.from_orm_with_request(
diff --git a/backend/tests/endpoints/roms/test_rom.py b/backend/tests/endpoints/roms/test_rom.py
index b8ceedb57..5e611f275 100644
--- a/backend/tests/endpoints/roms/test_rom.py
+++ b/backend/tests/endpoints/roms/test_rom.py
@@ -147,6 +147,7 @@ def test_get_all_roms_with_siblings(
     rom_a, rom_b = siblings
     assert [s["id"] for s in items[rom_a.id]["siblings"]] == [rom_b.id]
     assert [s["id"] for s in items[rom_b.id]["siblings"]] == [rom_a.id]
+    assert items[rom_a.id]["sibling_ids"] == [rom_b.id]
     # with_siblings alone must not pull in files.
     assert items[rom_a.id]["files"] == []