What I have learned is just like adults, not all teens are the same.
Our daughter sounds like your stepdaughter.. She has always been mature. She always could be trusted. I think one of the reasons was because we allowed her to be totally honest with us. We did not make threats we made promises. We taught her what we expected and told her consequences for the not acceptable behaviors would be.
We also told her what we expected her top priorities should be and that was her education. As long as she kept her grades up and was honest with us, we would allow her freedoms.. Mess with these 2 things and we will totally promise to mess with your life and your freedoms..
She upheld her part, so there was no need to yanking back her freedoms and our TRUST. She is now a double major in an amazing college. Always on the Deans list. We do not worry about her running wild. She knows what the expectation is.. She is actually harder on herself than we ever would have been because she has a drive and her own personal goals.
Children will make mistakes, they will falter and it is good. We want them to learn. But as long as they take responsibility for their actions and are honest about it.. I would not mess with what is working..
Where did we learn all of this, Our parents treated us this way. My mom always trusted me. I wanted to do my best and never have my mom be disappointed in me. My husbands family was also the same. They told him, "we know you know what is right and wrong." We lived up to this.
If it is working, why mess with it?