I had a close friend go through this exact thing. She did not eat pork for religous reasons, but the father's family did not respect this.
For one, let them know that pork isn't a necessary component to anyone's diet and can be replaced by any other meat. Let them know that Turkey sausage and beef hotdogs are just as good, and you might even consider sending them with your child while she is with them.
But more importantly your family is underminding your authority as a mother. This is very serious. If they can't respect your wishes then your daughter shouldn't go over there anymore.