I think he needs to know and you are right, he IS family now! He isn't just your FI's fam, but your's and he has a right to know. I like what has been said about waiting until after Mexico so uyou can have the wonderful drama-free wedding you deserve. Then let her know that you can't deal with it and you are telling him. Let her either step up or make you take the heat. Keep in mind that the saying "don't shoot the messenger" is there for a reason though. The FBIL will probably be mad at you. But the girl admitted that it wasn't the first time, so if I were you, I would def tell. We are grown-ups now (booooo, I know!), but we are and he should know what she does b/c it totally affects him.