Talk to your sister first, and tell her what's going on. Make sure you don't seem like you're accusing her of anything. If she cares about her family, she'll listen.

After you get a positive sign from your sister (maybe she can even help you talk to him), then talk to him. In private (not on the phone - this way he can't hang up), not in front of your family (except maybe your sister, if she agrees with you), state your problem. Lay it all out to him.