I can't stand my A's sister either. And I know the feeling is mutual! I'm just grateful she doesn't turn up at my house drunk! We ignore each other, haven't spoken for over a year and a half.
One thing I do always think is my A partner must've told her quite a few lies about me to cover up his problems. I know he used to make out to her that the reason he turned to drink was because of me and made out I was the hideous monster. She was so thick that she bought it all but he didn't help the situation. Don't get me wrong i still don't like her and can see how she tried to be manipulative and clever but I can see the part my A has played in our relationship. Do you think your ex may have told her a few lies to cover up what he does. So he still remains the sweet innocent little brother?