Do you mean connections to siblings?
I'd assume that siblings and cousins would be identified by their position in the hierarchy and not necessarily by an edge between them.
Definitely, having this edge between two nodes at the same level makes the problem harder, specially for edge routing that could lead to lots of crossings (depending on other possible connections you might want, like grandparent)
Well that's the difficulty, every time you have a wedding you are bringing in another tree, and the new spouse will have siblings and cousins that will have their own children and parents.
So I think a complete genealogy tree would be impossible to represent in 2d without crossing but most genealogy trees aren't that exhaustive so there must be an algorithm that creates a good representation when it is possible.
