Hi F.---I like what the other posters have suggested about setting down and discussing ground rules. FIRST, you and your husband need to sit down and discuss the situation together so that you are on the same page.
I really think that you might be able to offer her good advice, having just been a teenager yourself. Be sure to ask lots of questions to get her to open up about her feelings and her goals for her life. Use feel, found, felt when you respond to her answers, "I know how you feel, I felt..."
You seem like a very caring person and when you speak from your heart your step-daughter will be more likely to listen. But you also have to be firm. We always tell our kids that everything they want to do in life has to be earned. They need to learn that patience pays big dividends in the end. Again, you can refer to you own life. I'm sure it's not been easy at your age to have the responsibility of being a mother. Kids need boundries. Boundries give comfort to children as they learn to navigate a very scary world. Be confident in your responsibilities to your children.
I wish you much luck with your relationship with your step-daughter. It sounds like you are well on your way to knowing what you need to do. Be well, D.