I am planning my own wedding right now, and my fiance is not sure he is going to have my brother stand up for him. He is my only brother and fiance does not have any sisters or close female friends/relatives I could ask to be 'tit-for-tat' so to speak. I was a little upset, but in the long run, he can choose who he wants - it's his side of the bridal party. Though I do understand he's having her brothers, maybe he feels closer to them than she does to you. As a bride, I can tell you picking the wedding party is a lot harder than you think!
However, this girls sounds like a b*tch anyway, so you probably don't want to have to be around her all day anyway!!! I would mention to your borther that you would love to participate in his big day and could you maybe do a reading or something like that. Aside from that, keep being the great, supportive sister you have been - that's what is most important.
And, I agree with the others, if you parents are paying for the rehearsal dinner, they dictate who they will and will not invite. And proper etiquette is that bride and groom's immediate family are invited regardless (I say screw that out of town guest stuff - about 100 of my 150 guests are from out of state - I don't need to pay for a whole other reception on Friday night!!! Just my opinion!).