The second strongest predictor is a good couples' bond. Since the biological bond between parent and child predates the couples' relationship, the need to honor and respect the boundaries of this previous bond is essential prior to fully incorporating a stepparent as a major authority figure. Any shortcuts precipitate problems later. Oftentimes, because of cultural loading on mothers to be the primary caregivers, stepmothers are susceptible to being placed in this role precipitously. Do not take this role on.
Love includes discipline.
Your husband is failing to cope with parenthood. Perhaps the dynamic in his last marriage was to leave this part of the job to Mom and he is attempting to do the same here. But you are not the parent. Your stepdaughter has a mother and a father. Tell your husband you do not want to discipline his child, as it gets in the way of your forging a friendship with her.