diff --git a/Yattee/Services/Credentials/AccountKeychainCredentials.swift b/Yattee/Services/Credentials/AccountKeychainCredentials.swift
index ad822424..e78a1cb3 100644
--- a/Yattee/Services/Credentials/AccountKeychainCredentials.swift
+++ b/Yattee/Services/Credentials/AccountKeychainCredentials.swift
@@ -92,13 +92,22 @@ final class AccountKeychainCredentials {
 
         loginsByScope[scope, default: []].insert(account.id)
 
-        // Whatever was adopted here is gone, replaced by a login the user just
-        // proved they hold. Leaving the marker would keep this account out of
-        // the resync that follows the iCloud toggle, so the new credential
-        // would stay on this device and every other one would show an account
-        // it never receives a login for. It would also let a deletion retire an
+        // Whatever *this account* adopted is gone, replaced by a login the user
+        // just proved they hold. Leaving the marker would keep it out of the
+        // resync that follows the iCloud toggle, so the new credential would
+        // stay on this device and every other one would show an account it
+        // never receives a login for. It would also let a deletion retire an
         // unscoped item this account no longer has anything to do with.
-        markers.removeMarker(service: store.service, instanceID: account.instanceID, scope: scope)
+        //
+        // A sibling's marker is left strictly alone: signing a second login
+        // into the same instance says nothing about the account that adopted
+        // that instance's pre-revamp secret.
+        markers.removeMarker(
+            service: store.service,
+            instanceID: account.instanceID,
+            accountID: account.id,
+            scope: scope
+        )
 
         LoggingService.shared.info(
             "Stored \(credentialNoun)",
@@ -412,7 +421,12 @@ final class AccountKeychainCredentials {
         }
 
         store.delete(key: legacyKey)
-        markers.removeMarker(service: store.service, instanceID: account.instanceID, scope: scope)
+        markers.removeMarker(
+            service: store.service,
+            instanceID: account.instanceID,
+            accountID: account.id,
+            scope: scope
+        )
 
         LoggingService.shared.info(
             "Deleted the legacy \(credentialNoun) alongside its account",
diff --git a/Yattee/Services/Credentials/CredentialAdoptionMarker.swift b/Yattee/Services/Credentials/CredentialAdoptionMarker.swift
index 0ce00fa8..7982e750 100644
--- a/Yattee/Services/Credentials/CredentialAdoptionMarker.swift
+++ b/Yattee/Services/Credentials/CredentialAdoptionMarker.swift
@@ -109,9 +109,20 @@ final class CredentialAdoptionMarkerStore {
         save(all)
     }
 
-    func removeMarker(service: String, instanceID: UUID, scope: String) {
+    /// Retires the marker for one account.
+    ///
+    /// Scoped to `accountID` as well as the instance: an instance can hold
+    /// several logins, and only the account that actually adopted the legacy
+    /// secret may retire its marker. Matching on the instance alone let a
+    /// *sibling* sign-in clear it, which silently withdrew the migrated
+    /// account's claim on the unscoped item — after that its deletion could
+    /// no longer retire the legacy secret, and it could re-adopt it.
+    func removeMarker(service: String, instanceID: UUID, accountID: UUID, scope: String) {
         save(markers().filter {
-            !($0.service == service && $0.instanceID == instanceID && $0.identityScope == scope)
+            !($0.service == service
+                && $0.instanceID == instanceID
+                && $0.accountID == accountID
+                && $0.identityScope == scope)
         })
     }
 
diff --git a/YatteeTests/AccountCredentialsTests.swift b/YatteeTests/AccountCredentialsTests.swift
index ad11b1b2..5a1ab533 100644
--- a/YatteeTests/AccountCredentialsTests.swift
+++ b/YatteeTests/AccountCredentialsTests.swift
@@ -372,6 +372,33 @@ struct LegacyCredentialAdoptionTests {
         store.delete(key: KeychainCredentialStore.legacyKey(instanceID: instanceID))
     }
 
+    @Test("A sibling sign-in does not retire the adopter's marker")
+    func siblingSignInLeavesTheAdoptionMarkerAlone() {
+        let service = makeService()
+        let markers = makeMarkers()
+        let instanceID = UUID()
+        let adopter = Account.migrated(from: instanceID)
+        // A second login the user adds on the same instance.
+        let sibling = Account(instanceID: instanceID)
+        seedLegacyItem(service: service, instanceID: instanceID, value: "legacy-secret")
+
+        let credentials = makeCredentials(service: service, scope: "identity-A", markers: markers)
+        #expect(credentials.adoptLegacyCredential(for: adopter) == true)
+
+        // Signing the sibling in says nothing about the account that adopted
+        // this instance's pre-revamp secret.
+        credentials.setCredential("sibling-secret", for: sibling)
+        #expect(markers.hasAdopted(service: service, instanceID: instanceID, scope: "identity-A") == true)
+
+        // So the adopter can still retire the legacy item when it is deleted —
+        // matching on the instance alone silently withdrew that claim.
+        credentials.deleteLegacyCredentialIfOwned(for: adopter)
+        #expect(legacyValue(service: service, instanceID: instanceID) == nil)
+
+        credentials.deleteCredential(for: adopter)
+        credentials.deleteCredential(for: sibling)
+    }
+
     @Test("A login the user added themselves never inherits the legacy item")
     func onlyTheMigratedAccountAdopts() {
         let service = makeService()