This situation is a TRAGEDY waiting for a chance to happen. The girl is WAY underage, and her cousin is an ADULT who has a very unnatural, abnormal interest in her. He's very controlling, and that is one of the signs of a DOMESTIC ABUSER. Your GF needs to tell someone outside the family what her parents are doing to her, so that she can get some help to get out of the situation. Because if nothing is done, her cousin will escalate his behavior, and she'll either end up PREGNANT or DEAD.