A stepparent's role will change over time especially when it comes to discipline. At first think of yourself as a teacher on the first day of school. You are the adult in charge but your students don't really know you that well. So it's best to let the principal, their biological parent, set the expectations and standards of conduct. But as the school year progresses you will naturally get to know each other better. Good stepparenting is a function of relationship. Keep building yours.