Both my girls have used gauges in their ears. Both of them outgrew the desire for them on their own, without interference from me. What do you think might have happened if I'd made an issue of it??
To answer your question though, prepare to have her move in with you. Clearly she does not want to abide by JW rules, and her mum and step-father want to shove those rules down her throat. Have a sit down with her prior to her moving in and make it abundantly clear that while you are far more liberal than her mum and step-father, you are concerned about her safety and, as jamiebowers said, will not tolerate drug or alcohol abuse, you expect her to be responsible for her overall well-being (eating disorders) and particularly so when she is sexually active (birth control) and she has to take school seriously. Set curfews for her, give her chores and an allowance, and once she's old enough to get a job, she should get one.