Shawn, it's late here but I drop this idea, maybe it does not fly at all, but let's try.
Define P-Node (Parent Node) as the set of one person X and all her/his children (0+)
The P-Nodes of X and Y are merged iff X and Y are spouses AND the set of their children are equal. In this general case they have the same structure as your famly Nodes.
All the corner cases seem to boil down to have P-Nodes with a single parent, sharing only a part of their children, hence not merged
Put an edge between two single-parent P-Nodes if they have at least one common child and/or if they are spouses.
Seems to me from my scribblings that does not create more nodes than your Family Node model.
Siblings are grouped in the node(s) of their respective parents in all cases, hence neither artificial distance nor useless clique. Brigham Young sits in a big single parent P-Node with his many children, each of his wives in their own, and only 56 links.
I'll let you make a beautiful drawing of this.