Please read Combatting Cult Mind Control by Steven Hassan asap. Here is a lnk to his website: http://www.freedomofmind.com/
I was also 16 when I met my jw boyfriend and converted to marry him, which I did by 17. It was a nightmare marriage where I was so seriously abused and my life threatened that he was twice forcibly committed to mental institutions. Watch Tower doctrine dictates that remarriage is impossible unless one spouse can prove adultery against the other or if there is a confesson. I was badgered by the elders to remain in the marriage and given the choice of either stalking the man who was threatening to kill me or excepting the fact that he would stalk me.
Unfortunately my family were also jws, and I was badgered by my own mother, and I made what was probably a false confession of adultery* just to keep my ex-husband from having one more second of influence over my life. In doing so, I was df'd and lost my home, most of my possessions, and my family.
The Watch Tower truly is a cult, and even their recent literature encourages women to stay in abusive marriages. See the 2/15/12 Watchtower that will be studied this coming Sunday. Do whatever it takes to get your niece out, but do it correctly. Good luck!
*This was more than 20 years ago, and I'm not positive when I started dating again.