I would say that her boyfriend is a VERY insecure man. He needs to feel important and absolutely necissary in all situations. I have heard of relationships that start out this way, and turn verbally and physically abusive. I would hope she would want to get help fast, and not be ok with the situation at hand.
If my man tried to do this I would first probablly be angry, but try to analyze what is causing his insecurities, and help him out.
Hope your sister takes care, and everything works out :)