Sistershelp,
I wish there was more I could add to what Marian said. Perhaps others who are more experienced than I am could... but I just wanted to say that, as long as you know the situation to be what it is, why not just try to genuinely enjoy this time you have with your neice? I know how badly you want to protect her, but just giving her the positive influence now is undoubtedly doing her a world of good, in so many ways.
One thing I do know about all children is that they can smell our fear, anxiety, etc... and not just when it's towards them. You say now what? I say try to be as happy as possible that she is there with you today, and enjoy having her there today, and don't worry about tomorrow, because it will come whether you worry or not, and you've done all else that you can.
*hugs and prayers*