Preciousjeni I've been in your shoes. You have to abide by your future sisinlaw's wishes as to her bridal party. Leave it alone. Considering it a blessing that you don't have to where some ugly monstrosity she will no doubt pick out. Even if you're not a bridesmaid you will have a front row seat and you're still be his sister after the wedding. Do not get your brother involved as he is going through enough stress with her and her family.
Rise above her witchyness and be a place where your brother can vent. Remind your parents that this woman will be the future mother of their grandchildren and she will call the shots as to when you get to visit. You're doing the right thing by defending her although I'm sure it's not easy.
As for the rehearsal dinner if your parents are paying for it, they will call the shots as to where it will be held and who goes. On this matter your parents should remind them that you are part of the family package.
Try and make it fun for your parents. Tell your brother that it will be your responsibility to make sure mom and dad look their best for this day. Go with your dad when he gets fitted for his tuxedo. Arrange his appointment for haircut and shave. Go shopping with your mom and get yourselves some killer dresses. Make arrangements to get manicures, hair, and makeup done together.
Good luck.
