your girlfriend should never have put you in that position where you half to choose your family over her she should think about you and your feelings if she cares she will respect and think of you in this situation and respect your family even if they dislike her and if there disrespecting her tell them how you feel it isnt right to do when she isnt doing it back but i wouldnt break up with her honestly this is something shes gonna half to get over especially if she cares about you.
Firstly breaking up with her is not the answer. Talk to her and see what the problem seems to be. Once you've done this give advice like, Well maybe if you address this with your parents you guys can work it it some how." but be sincere when you say it. and when you're around her family bring up topics everyone agrees on so then they'll see its not so bad to just talk.
Assuming you mean the GF & her family aren't getting along, this isn't your issue. It's pre-existing. As much as you may care for her and as badly as we all seek world peace, sometimes it's better that families take a breather. They'll find their way back to each other, families have a way of drifting towards each other in the sea of life...often without realizing it.