I understand that you generally shouldn't rag on your girlfriend's family, but my girlfriend's sister has been a right bitch to me for the past year or so of us dating, and it is past old. She doesn't do anything about it, and when I finally tell her to shut her mouth, she gets mad at me. What do I do here?
This. My boyfriend's sister's the same - she hates me more than anything even though I've done nothing to her. I just avoid being around her, and I make an effort to be civil and polite if I do have to be around her.
A difference between my situation and yours is that my boyfriend does stand up for me against her (which probably makes her hate me more). I guess you could talk to your girlfriend about it, but it might be hard for her to stand up for you if she has a good relationship with her sister. My boyfriend's sister is a bitch to him too, so he doesn't mind it, but I do think it'd be more difficult for him if she was nice to him
I figure that if avoiding her sister complicates their time together as a couple, that deprivation will light a fire under his gf's ass to stand up against her sister. Not to make it sound manipulative but action sometimes requires a pain point.