Well, here's a [link|http://www.focusas.com/Resources.html|LINK] to a lot of links that may (or not) be of value. Without question, I suggest trying to get help from professionals, starting with a county Social Services agency.
You cannot consider just trying to get by the next few months and then cut off all contact. This is your wife's child and you have to do whatever it takes to get your step-daughter through this very difficult time. "For better or worse...", remember?
Unfortunately, none of us are trained to deal with even the typical transformation of a child to an adult and you obviously have a case much more serious than that. I have parented 3 children into productive adulthood and I more attribute that to luck than to any skill.