Unless there's more to the story about your girlfriend that you haven't said, I agree with what Bill and D&L and most everyone else said in response. Your family sounds hostile and is probably intimidating to your girlfriend. Perhaps that's why she looks bleak when she has to sit next to your sister. I think Bill asked an excellent question: do you ask your sister to "comply" as well? And what do you mean by comply? Do you mean act the way your family prefers? I think you should be more emotionally supportive of your girlfriend. Best of luck.