I know saying "no" can be the hardest thing for some of us to do, particularly with family that has figured that out and takes advantage of it. One thing I learned is that the only person I really have the power to change is myself. If I sit around and wait for others to change the way I wish they would, I'll be waiting a LONG time.

I know you care about your niece, but you can't help her if being around her triggers you the whole time. I could be wrong, but I can't help but think your family is pawning her off on you so they don't have to deal with her for awhile. It's a bad situation, but they are going to have to handle their own child. You seem to be a bit of "rescuer" where she is concerned, but the rescuer can't save anyone if the person they are trying to rescue is pulling them under.