This is going to sound very harsh but you should do nothing. She's not 16 anymore, at least I hope she's not if she's been married for 10 years. The simple fact is that some people make poor decisions, time after time again, and aren't smart or savvy enough to wise up to situations. I know she's your sister, and I can't imagine how tough it is to watch her live her life this way. At the end of the day she's a grown woman, she's made her bed, now it's time for her to lie in it. If she asks you for help getting out of the situation then do what you can. But if not let her live her life, no matter how ridiculous you think it is.