How to deal with a rude best man?

Ah, yeah. Twins are really difficult to get between. I'd be pissed too if someone wanted to marry my sister. I would definitely address the issue, but don't act as if the blame is on him. Perhaps talk to him more, casually, mostly about his brother. Remember--he had him first. He has a right to be irked, but that doesn't mean he has a right to be rude. Listen to him when/if he tells you what he thinks, and plan something, the two of you, so that everything will work out. Also, I wouldn't so much ask your fiance to speak with him, because it isn't he that has the problem. Twins talk (trust me on this one), so he WILL find out if you put his brother up to talking to him. Do it yourself. Good luck, I hope it works out, and I hope we helped!