she just doesn't care about me anymore.

It sucks, but that seems to be the case here. It would be one thing if she were reacting to another's ignorance (the episode of The Office referenced in another thread here is a perfect example of it), but she is clearly accepting intentionally homophobic behavior. Benign ignorance is fairly easy to fix, homophobia is not.

In the end, you can't make her become a better person. You can't control her reactions. You certainly can't control what your BIL does. You did the bigger thing and put up with it for years hoping it would change, but there had to be a breaking point somewhere. Don't think of it as you choosing to do something so much as you being forced to do something because of BIL's malice and sister's apathy.