Why can't a post have multiple parents?

I uploaded post #4847005 as a child of post #4846860. The latter is the parent of all five single character illustrations of which it consists, which helps people find them. But post #4847005 is actually an edit of post #2488189 (by the same artist), so that one should be its parent.

• Any suggestions what to do in this situation?

This situation made me think that maybe our current parent/child system is flawed. post #4846860 should really be the child of all five single character illustrations, since they are the originals and it is a derivative of them.
Niconico has a "content tree" which allows a post to have multiple parents and children. That way a derivative work can link to all its sources and can in turn be linked as the source for any number of other works. I think this could be a good idea for Danbooru.

I changed the relationship so that post #2488189 is the highest child, since it's the best version of that picture. The two posts are basically identical save for the transparent background.