In that scenario, the children automatically have a lateral relationship by having the same parents.  However, if you had spaces for the children's aunts and uncles, they could be shown as connected to the children's parents by a  sibling feature.  All the inter-relationships do get a bit complicated if you think too hard about them.  That's why I said that simply showing a connection is good enough.  In your example, it may be useful to show the connections of the children's aunts and uncles to the children's parents.  As in real-world scenarios, just because someone is your aunt or uncle, doesn't mean you have to automatically like them or share any of your private information with them.