You need to protect yourself and your family .. Its sad that your sister is making such horrible decisions about her life . But you cant force her to get help but you can stop having any contact with her.

Is the father of her kids taking care of them ? Are you just helping out? When she blows in and out of town does she see her children? I would think that the father would not allow that because he is trying to provide a stable home.

What a difficult situation to be in.. take care of you and your family and hopefully at some point your sister will seek out help for herself.