I became engaged in July and I knew right away who 2 of my BM's were going to be. I then decided to ask my brother's girlfriend to be the 3rd BM. We get along well and the whole family loves her. Here's the problem: my brother and her are having some major problems and I don't see them making it much longer. My heart breaks for both of them but I'm also concerned since she told my mother that she would step down if they broke up. I'm ok with that because I understand it might be awkward. I have another friend in mind to take her place but how do you ask someone who knows that they are just a backup? Our wedding is 10 months away and the dresses have been bought. I'm really not sure how to handle this whole situation. Any advice?