OH BOY!!! I can't even begin to offer you much advice but I just wanted you to know that i'm so sorry that you are in this situation. The only thing I can think of is to maybe pull her aside (and maybe one person that can back you up or just be your witness; preferably someone who sees your side and someone that she will listen to...I wonder if she acts like this because she feels she is loosing her brother...), sit her down and just tell her how you feel without making it seem like you are attacking her. Like it or not, you guys are going to be family. And if after that, she doesn't knock it off, then take your fiancé’s advice and just ignore her. That will be hard but you will just have to hold your head high and take pride in being the bigger person.

PB