That situation really sucks! I have to agree with the previous posters that firing her as a bridesmaid would only create more issues and while it may give you an initial satisfaction, it sounds like it would cause you more pain in the end. If you can have a one on one with her great, if not, I think Inked1's suggestion of letting her make the decision is the best way at this point. Send her the necessary info, but if she is not responding then don't go out of your way. Let the ball fall into her court and the decision to act like an adult be up to her.
I hope it all works out for the best, but the good news is your FI sounds very supportive and you will have a wonderful wedding regardless =)