Well let me be the dissenting vote here...don't back out of being in the wedding.
Have a sit down (perhaps lunch) with the BTB. Explain how excited you are to be in the bridal party and you had certainly planned on participating in the shower, bachelor party and whatever other events are connected to this grand event.
HOWEVER, you were told one thing then another. You have not been kept in the loop about finances, locations, planning, etc. Be sure to emphasize your joy with being in the wedding and wanting to do your part with the other events.
This is a woman that is going to be in your life - probably forever (even if they ever get divorced and I'm not hoping that they do). I think you are well served to bite your tongue now and not add drama that may well follow you the rest of your life with your brother. Again, tell her how excited you are to be part of this wonderful wedding.
And I am NOT saying that anything is your fault, just saying you might want to hold your tongue with a pair of pliers and clear the air with your new SIL to be now so the future with your bro at least starts off peacefully.
I agree with this.
I think dropping out of the wedding party is the nuclear option. A last resort.
She left you an opening, that what she's hearing from you doesn't match w/ what she's hearing from other people
So FIRST talk to your brother. Tell him everything, in a non-accusatory way. Print out an email from the MOH if necessary. Tell him that you want him to know that your attitude is, "I'm exciting for you, happy in your choice of bride, was eager to be part of things--so eager that I sent my budget info to the MOH right away!"
Then tell him that you're going to sit down with his bride and explain these things to her as well, but that you wanted him to know straight from you.
Then call her--don't text, your instinct is right there--and say that you're concerned she's not hearing the same thing that you're saying, and that you don't want to communicate with her through a middleman. Can you meet?
Then say to her, "I'm happy for the wedding, and I don't want this unpleasantness and imprecision to sour things." Then lay out the timeline. Print out emails if necessary.
And tell her that no matter what someone else is deciding based on their communication with you, your own attitude, straight from you to her, is that you are happy about the wedding, willing to participate, bummed that the party was rescheduled so that you simply can't make it.
And then say, "I don't want to be where I'm not welcome. If these things, and my explanation, mean that me being in the wedding party is a negative thing, then I absolutely will step down, no hard feelings. You should be happy with your bridal party. If you can't be happy with having me in it, of course I should step out."