Bridal Party Issues, Help!?

This has not been a very long engagement, and I told my bride's maids from the beginning that if they agreed to be in my wedding, I needed their help. Now my maid-of-honor has become rude to almost everyone, and just plain unpleasant to be around. Her boyfriend, who happens to be a groomsman, is not helping at all. he's being as much of a diva as she is. She now refuses to help with any pre-wedding planning, including shopping for her own dress. They are both demanding that we change other party members and guests because they don't want to be around those people. I'm already up to my ears in unneeded stress, and don't need their self-serving attitudes. I've talked to my fiance' and he says he's doesn't know what he wants to do about it. The thing about this that bothers me the most is that she is my cousin. My own family being so selfish that she will not help me or hold her tongue and act like an adult for a few hours so that everything can go smoothly. The wedding is at the end of April. I want them both out, but I don't want any hard feelings, and I don't know if I can replace them on such short notice. What do I do?