Thanks for all the suggestions. My sister is an intellectual to an extent, but she veers away from self help material.
At this point I'm just trying to focus on what I can do to help without playing into the controlling sister role he is trying to box me into. I have faith in our sisterly bond, I almost want to laugh if he thinks he can come between us, but then part of me does still worry.
From some of the reading I've found on the subject, it typically takes 7-9 tries to break up with a controller. She is at 2. Long road ahead I guess.