Um, I'm not going to get involved with this drama over already existing fan characters -- but let me through my hat into the discussion on your fic idea.

Personally, I've been around the block a few times when it comes to fanfiction. And in my opinion, the sister/brother/twin storyline is done to death. I find that it's always a good idea to stick with canon characters. OCs are perfectly fine, as long as their own stories muddy up the pre-established canon. Then again, I am a canon freak (can't help it), so my opinion counts for only a small percentidge of the fanfic community.