Cinders, Continue to be the person you are and that's all anyone can ask of you. You owe her nothing...being polite and considerate is enough....for your children's sake. Yes, she is struggling and you've no hand in that struggle. It is a struggle that she created and she has to be the one to figure it out, or should I also say your h should be helping her.
Enjoy the visit w/your sister and I hope that your niece is doing much better.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.