Wow that's a tough situation to be in. I mean I would probably tell her that if she wants to be there you would appreciate her coming but you don't want her to ruin your wedding, and that if she can't be nice she should keep her comments to herself because this is your day not hers. The thing my mom has always said about my sister's husband and my fiance when people ask her if they like them (which fortunately she does) is well "yes I do, but I don't have to like him because I'm not the one who has to live with him". So remind your mother that you love this man and want to marry him, and it doesn't matter what she thinks because it's not her relationship.