Choosing your Best Man or Maid of Honor is a complicated decision. I've always thought choosing your sibling is the safest choice because you don't have to decide which of your friends you trust the most. But at the end of the day, it's your wedding. You should be able to choose who you want in your wedding party and who you don't want without incident.
One groom is in the unusual predicament of wanting his brother's middle school bully to be his Best Man. He developed a friendship with this former bully in college and considers him to be his best friend. The brother is so angry about this choice he's refusing to go to the wedding. On the one hand, I think that if you're in your thirties, you should have processed the trauma of middle school already. On the other hand, he has a right to feel upset since he wasn't even asked to be a groomsman.