That is definitely a tricky position. I wish I had an easy answer. I was in a similar situation with my ex-sister-in-law. She was a total monster...cruel and heartless to me but an angel in front of him. In fact, he would get angry at me if I stood up for myself and blame me for any tension! She even admitted she enjoyed manipulating people like the puppets she thought they were. I tried to tell my ex for years...and it didnt do any good.

I guess the point I'm getting at is that you can not make someone see what they don't want to. It took her personally attacking him to wake up and see what I had been dealing with for 12 years (by that time). IMHO, I wouldn't stand in the way of her relationship with her sister, but I would refuse to allow her to affect you and the other members of your immediate family. She will not see what is going on as you do until she deals with whatever issue she has with her sister. Sadly, that's not something you can change.

Best of Luck!

~Seanachai~