Keep talking until she gets it. I was never in a marriage
like that but I was abused by my boss. We were together
for more time than he was with his girlfriend. He made me
afraid just like your sister and I was stupid enough to stay
and take it. Get her to a shelter if you can before it is
to late. Get her counseling and help her wake of to the fact
she doesn't have to take that from anyone.

Good Luck