Can I Be In Two Places At Once?

Ever since I met my boyfriend's family we have had difficulty getting along. Mostly there have been issues with him brother, who "didn't like my attitude" before he had ever spoken to me. But it's okay, because he is an a$$, and I don't like him either.
So my boyfriend's brother is getting maried, and my boyfriend is an usher at the wedding. When I asked him if I had to go, him and his mother put up a fight saying I had to. My argument is that they don't like me and don't want me there anyways and I fail to see why I should be somewhere I'm not wanted...and have no interest in being whatsoever. So, I proceeded with the jokes, "I'll find something better to do," "I'm working that day," "I can't go..busy!" and so on....
Except...
My cousin's wedding is the same day!! So now I actually have a reason to not go...
...but I only found this out AFTER I agreed I'd pull down my stubborn wall and go to the brother's wedding.
Now, I don't know what to do. I have known this cousin since before I was born, and would feel horrible if I didn't go.
WHAT DO I DO!?