Wow. You have every right to be angry. That is just inexcusably rude. If it happens again, I would cut them off as soon as they even begin to hint that your brother's fiancee is mean. Just say something like, "You are perfectly entitled to your opinion, but I would appreciate it if you would not criticize my future sister-in-law in my presence." If they continue, just repeat it. If they still don't respect your polite requests, I would remove myself from the situation. Best wishes to your brother, and good luck dealing with your friends! Oh, and kudos to your for having much more tact than your friends have.