If it was me, I would go ahead and let them be the bridemaids. I would also have a talk with your soon to be husband. Your bf's ex is trying to control him, and it seems to be working. I would tell him that if he doesn't get to see his kids that he needs to take the ex back to court and get his visitation rights. What does your bf say when you ask him to talk to his kid? I would encourage your bf to talk to his kid. You should try to get friendly with her, even if she doesn't want to. I'm divorced and my ex has talked so down about me to my kids that I had to show them differently.
I know that it's hard. Just keep trying and hold your head up high.