You know, I think I would post this one with one of the docs on the board. Find out what they think about it.
He is too old to sleep with his sister, due to hormonal changes and all, even if there is no chance in hell that he would do something, it could be used against you.
Perhaps you can sit and read to the little one in her bed until she falls asleep? Start a new night-time routine at your home, lots of fun, books, stories you make up, etc., to keep it light and non-scary. The little one may have her imagination running away with her and habits for them are hard to break...I'm trying to get my 3 yo dtr to sleep in her bed instead of mine! LOL So far, NO LUCK!
Good luck....nosonew