Iceman, if you don't want to go, don't. The worst thing you can do is end up at a wedding, miserable, and bringing everybody down. And if this groom is as much as an asshole as I see it, then who knows if there may be any conflict, and the last thing you want to do is make a scene at your cousin's wedding.

However, if it was me, I would go, simply to see my family, notably my cousin on her special day. She's obviosuly happy with him, at least for now, and it would make a great time to reunite with your entire family as a whole. I wouldn't socialize with the groom if I didn't want to, but I would surely show up to congratulate my cousin on this, even if you don't agree, special day.