If a child has an emotional problem, the whole family has an emotional problem. This is the basis of family dynamics. Start with looking at theraputic parenting, see if you can find anything useful, I hear there are some good videos and podcasts around the web advising on dealing with difficult teens.
Try to get some family therapy going, make it clear to your stepdaughter that family therapy is so that you and her father can have help to hear and understand her, rather than to deal with her issues.
Make friends with her, can you do martial arts with her once a week? its always good to learn self defence and it may help your stepdaughter in the future, if she doesn't learn good self care.
Im decades down the line with this, (although i didn't bring up my stepdaughers, their parents died two decades ago) What I can tell you is that if this isn't dealt with correctly it will echo down the generations. Im watching exactly that happen right now, have had to disengage from the next generation for the good of my own mental health.
Modelling the behaviour you expect her to display is always positive, learn as much as you can about personal boundaries, model those too, while encouraging dsd to practice her own boundaries, help her learn to be comfortable with having her own boundaries. Practice it together, pointless ones are great fun, make it game between you two.