Well don't lose hope on hearsay. And don't allow yourself to feel guilty for the choices your sister has made. It sounds like she has made some bad decisions but they were her decisions and at the end of the day, the children's wellbeing is more important than protecting your sister. But I know you know that.

If you can, I would get in touch with your sister, tell her you love her but that she has to take responsibility for her actions ... and you will be there for her 100% when she wants to turn her life around. But until then, the kids come first.

Good luck! I haven't been in your situation exactly but I totally know how sucky it is when someone close keeps *****ing up.