HI Horse Girl,
I wish I could say there is a "right" answer to what you should do, but I'm afraid there isn't. I think so much depends on you and your relationship with your sister and the family. If she is addicted, she will stop when the pain of using is worse than the pain of getting clean. For some that takes loosing everything, for others, there are low bottoms. The most important thing for you is to take care of yourself.
I think if it were me, i would try a conversation with those closer to her, like her husband to see if your sister would be receptive to speaking with you.
Keep reading here; there are great people and lots of support.