You could maybe point out that you need someone to hold your bouquet while at the altar! Maybe she could just be there at the side for you, she doesn't have to walk down the aisle - organise the job of BM to suit the pair of you. If it's important to you then talk about it, you dont want to regret any part of your day. And make sure you're happy with the final outcome so that when it all happens you can smile and be happy, no matter how the two of you resolve the issue.