without reading any other replys, my first instinct answer is let her move in with you if she wants. She is being abandoned by her father, and he is telling others to abandon her too. That's really disgusting and cruel. Don't let their messed up logic get in the way of an obvious anwer. If you love her like a daughter, then find out what she needs right now, and if it's a home, I'd let her move in.
Also help her out with some therapy. My friend did try to kill herself in February over a breakup, and her parents, though not JWs, treated her as if they were JWs. She is also still Obsessed with they guy, but thank goodness, she found some free therapy and psychiatric care (including meds.) YOur niece needs to get back on her feet, and only sane people, like yourself, can help her do that.