Pretty much all you can do for your parents is listen to them an try to be supportive. At least for me was harder when my other kids would get mad us for stressing out over what their AD sister was doing. For yourself you need to try an detach hard to do I know. I do understand them dragging their feet on filing the papers, but they really should go ahead an do it. Once her little girl is safe they can work on letting her fall with out the added worry of what will happen to your niece.
Sorry you are having to deal with is for some reason our AD always seemed to act up more around holidays, or birthdays, don't know why.