I had some experience with this. In my case my brother exes and their wives and girlfriends were very jealous of everyone who even look at my brother where one of my sister who has a friend who is extremely controlling and is jealous of anyone who has my sister attention It kind of sound like your girlfriend might have a problem with your family and is trying to tear you apart from your family. Have you ask her why she is acting the way she is around your sister? Have you thought about gathering evidence like have a friend get together with your sister and girlfriend while your not there and record without their knowledge to see what is going on? Or have you listening on a phone call so you can hear what is going on. I would reevaluate my relationship with the girlfriend.