diff --git a/tests/static/nodes_subpackage/subsub_sibling/__init__.py b/tests/static/nodes_subpackage/subsub_sibling/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/tests/static/nodes_subpackage/subsub_sibling/demo_nodes.py b/tests/static/nodes_subpackage/subsub_sibling/demo_nodes.py
new file mode 100644
index 00000000..533e1005
--- /dev/null
+++ b/tests/static/nodes_subpackage/subsub_sibling/demo_nodes.py
@@ -0,0 +1,16 @@
+"""
+A demo node package for the purpose of testing node package registration.
+"""
+
+from typing import Optional
+
+from pyiron_workflow import Workflow
+
+
+@Workflow.wrap_as.single_value_node("sum")
+def OptionallyAdd(x: int, y: Optional[int] = None) -> int:
+    y = 0 if y is None else y
+    return x + y
+
+
+nodes = [OptionallyAdd]
diff --git a/tests/unit/test_interfaces.py b/tests/unit/test_interfaces.py
index 87442fa1..4281ed4d 100644
--- a/tests/unit/test_interfaces.py
+++ b/tests/unit/test_interfaces.py
@@ -44,6 +44,7 @@ def test_registration(self):
         self.creator.register("static.nodes_subpackage", "sub")
         self.assertIsInstance(self.creator.sub.demo_nodes, NodePackage)
         self.assertIsInstance(self.creator.sub.subsub_package.demo_nodes, NodePackage)
+        self.assertIsInstance(self.creator.sub.subsub_sibling.demo_nodes, NodePackage)
 
         with self.subTest("Test re-registration"):
             self.creator.register("static.demo_nodes", "demo")