OP, for your brother's sake, go to the ceremony. Just sit in the back of the church if that's all you feel up to doing. Let him know you're there. Because his life is going to be hellish.
I'm sure he's having major doubts by now, and he's not going to tell anyone because he doesn't want to look like the bad guy. She's maneuvered it so that he has to choose between her and his family, and he's going with her because he knows there's be living hell to pay if he doesn't. He figures he can't back out of the wedding because he knows how much money your parents sunk into it, and besides, he'll look like the dirtbag who abandoned his dear, sweet fiancee.
Your mother is just trying to keep the peace and keep the engagement/wedding from imploding. She may come to her senses and realize that that's the last thing she should be doing.
My heart breaks for your brother, because I know someone like the fiancee. It's all about her, all the time, and God help anyone who doesn't do things her way, or give her everything she wants. It sounds like she's driven most of her own friends away, and over time, she'll drive your brother's friends away, too. She's already driven a wedge between you and your brother, and between him and his niece, and your parents are going to be next. She'll isolate him, and she'll beat him down, until he does whatever she wants just to keep the peace.
I'm sorry. I hope he comes to his senses before the wedding.