Oh dear. I totally feel for you. My Future Sister-In-Law is also seriously psycho, in some of the same ways. Here’s our story:
In our case, Fiance is the one with kids, from his first marriage. Although his sister introduced him to his ex, she decided soon after he asked her to marry him that she wasn’t good enough for him. She pitched a fit and refused to come to the wedding, causing a huge family fight. She then proceeded to treat his ex so badly for years that eventually they refused to bring the kids to see his parents if she was going to be visiting too.
After his divorce, she was all lovey-sweet and supportive to him. He got into the habit of spending most holidays with his parents, and she was always there. She’s not actually very nice to him either, and valued the kids primarily because they were a great audience (everything is about her) so now that they are teenagers and less interested, she’s not all that nice to them either. At least everybody pretty much understands where she’s coming from, so nobody seems to think what happens now is my fault in any way.
Which is, in a nutshell, that she can hardly be in the same room with me. She bursts into tears almost at the sight of me. She locks herself in her room. She constantly asks her mom and my Fiance "Does SHE have to be here?" Even for Christmas, when they came to his house, it was "Does SHE really have to be here?"
Luckily, that finally really pissed Fiance off. He sat down and had a short but emphatic talk with her. He let her know that he had made a mistake all these years letting her get away with the way she treated his ex, and he wasn’t going to continue that mistake. He told her that her choice is simple – she can learn to treat me appropriately, or she will no longer be welcome in his house. He dropped the bombshell that we are actually selling his house and moving into mine, which really floored her. He told her that he hoped that she could be happy for him, as he was very happy with me, but if she couldn’t then he could accept that. But he wouldn’t hear any more about it.
She actually told him (here is the seriously psycho part): I always thought that when mom and dad were gone, it would be okay, because I would always have you. He told her (he was totally floored, I think) that after their folks are gone, she will still have him, and me, and the kids. Unless she can’t somehow learn to be friends with me, in which case she won’t have anyone.
Pretty harsh, and I think rosychicklet’s suggested words are probably better, given that I don’t think your Future Sister-In-Law has the history that mine has. But your Fiance does need to talk with her. He needs to let her know that he has chosen you and your child, that the three of you (and any more kids that you have) will be a family. And that whatever issues come up, the three of you will deal with them as a family. And while he hopes she can be happy for you all, she really doesn’t have to worry about things like where you will all be able to live – because that’s your concern. Not hers.
I do think you need to reconsider the idea of having her as a Bridesmaid or Best Man. She has already indicated she is less than supportive of your marriage. I hope that she is not the type to make more problems, or even cause some kind of drama on your wedding day, I am concerned. If you and your Fiance can’t get this resolved pretty quickly, I would also suggest that you let her know you don’t feel its fair to ask her to stand up for you, given that she clearly has doubts about your marriage. Once she is only a guest at your wedding, you will have to worry much less about what is going on in her head. Although I still worry. I have let Fiance know that I have a couple of girlfriends designated to go ahead and politely remove her if she can’t manage not to create a scene. Because I’m pretty sure that she will.