Oh, man. I'm sorry. I would be really angry if this lady (and I use the term loosely) was trying to join my family.
However...I don't know if there's anything you can do about it. Your brother obviously sees *something* in her, although I can't imagine what. If I were you, I would talk to him and tell him that you are concerned for his sake. It might not do any good, but it's worth a shot, right?
But even if worse comes to worse and they do get married, there's hope--my brother-in-law and his family absolutely *hated* his stepmom, with a passion, and he never could understand why his dad was with her because according to what I heard, she was very controlling and witchy. Well, evidently his dad got to the point where he'd had enough...so he divorced her, and now everyone is much happer.
So see, sometimes people do come to their senses, even though it might take a while.