I was setting up my legacy tree and was at a bit of a loss initially how to manage my multiple jedi characters of different species. The "Child" and "Adopted Child" options, while nice, didn't really appeal to me since Jedi are not "supposed" to engage in romantic relationships and bear children. And having everyone stretched as linked "Allies" is flat and boring.
I was going to suggest a new relationship assignment of "Mentor" - that would be a sub-level assignment similar to a "Child" or "Adopted Child" that represents a character that has received training, guidance, or other assistance in their development.
So just a suggestion - Add a "Mentor" option that could give logical "lower level" legacy tree slots to characters, since having everyone as an ally or sibling doesn't give feel as a hierarchy.
Again, this is not a criticism on the existing system, and I realize that even from roleplaying standpoints, there are many ways to explain why your Twi'lek Consular might be the child of your Human Sentinel. Just food for thought