Maybe your stepdaughter is acting out - she's not too old to do this. Having a stepfather leaning on her because he is lonely and his girlfriend has not been available can cause a lot of confusion and agitation for a child or teenager, or even a grown up. Filling that gap is not the role of a stepdaughter or daughter. It's crossing a boundary and putting an expectation on her that I would think would be uncomfortable and confusing. Plus, since you are her stepfather, maybe she had a difficult relationship with her father, as well, accounting for her behavior in high school. Is that possible? Does this make sense to you?