My girlfriends parents dont really like me that much

I’m having this small problem with my girlfriends parents. Its not that there bipolar which they are. its that they don’t like me. I know this b/c of the fact that they are really nice to me to my face right and then I hear them backstabbing me on the way out the door. When I’m on the phone with my girlfriend I can hear there arguments in the other room about how they cant trust me. Then when I’m with them they tell me they trust me and they would do anything for me but when im gone they say I’m terrible. Her parents favor one of there daughters(which theres two of them) and then leaves the other one(who is my girlfriend) out of the picture. I don’t like them but I’m trying to hold everything together. Im trying to gain trust in there family, try to keep my girlfriend from taring there eyes out (figuratively speaking), and my family hates them which doesnt help at all so im just trying to stop a family feud b/c I do not want to end up like something out of romeo and juliet. it maybe my favorite book but I do not want to be like that. Help please