It's not really up to the bride and her mother who gets to go to the rehearsal dinner. (a) Etiquette says all immediate family members of the bride
and groom should be invited. (b) Remember the Golden Rule: He who has the gold, makes the rules. If your parents are paying and they want you there, the bride and her family have no right to say no.
A few people have commented along the lines of why would you want to be a bridesmaid for a bridezilla who has treated you so horribly to date? and also that the bride picks her attendants, the groom picks his, and although your brother might want you to be a bridesmaid, it's ultimately up to the bride whether she asks you. These are both good points. That's why I made the (admittedly unorthodox) suggestion of your brother asking you to be one of
his attendants. You'd stand on his side and possibly wear a dress that's different from what the bride's attendants wear. I've never seen this done, but I have heard of it.
(hee hee... then her brothers could be "bridesmen". Won't they look cute in big poofy bright orange taffeta gowns with huge pink butt bows?? LOL
)
Or, if that's just too weird, there are so many other honors you could be offered if the bride is dead set against your being a bridesmaid. Talk to your brother and let him know how hurt you are... then see what develops.