You are 100% right....you don't need this drama. You are under enough stress.
I agree with ItGirl, try talking to her and let her know how much she means to you and explain that you really want her a part of your day. Explain to her that you don't have much time and you have to get on with your plans so you truly need to know one way or the other. Set a deadline and then give her space to think it out. After that, as hard as it is, you'll have to move forward with our plans....with or without the siblings.
I'm going through basically the same thing....my bridal party has changed multiple times because of family issues....I finally had to cut my losses and move on....It's my day....just like it's your day and you deserve the best.
Best of luck...I really hope it works out the way you want it to. In either case, I'm sure your wedding will be beautiful and something you'll always cherish.