Maya supports generalized instancing. Generalized instancing means nodes which instance another node do not have to be siblings.

Node 2 and Node 4 are not siblings yet they each instance Node 3. More complex hierarchies can be created, so long as a reference is not made back up the hierarchy. Doing so could create a cycle which would break the acyclic nature of the DAG (remember that a DAG is a directed acyclic graph).