What would you do about this maid of honor?

I think you need to have an honest, frank chat with the MOH and get things sorted out. You have as much right to be a bridesmaid as she does and I think she should be asking for a bit of input from the rest of you guys. Don't start a riot just ask and say what you need to and if the outcome isn't how you wanted then just carry on as you are, go and enjoy being a bridesmaid for your brothers big day. Don't let this person ruin your day. Enjoy.