Legal Question in Family Law in California

Your step father derives his authority over you through your mother. If your step father is speaking for your mother - then yes you have to listen. If he is telling you to do illegal or dangerous that's a different story. This is not a legal question so much as it is a problem with the dynamics in your family. It could be something as simple as regular "family meetings" where everyone in the house meets to discuss issues that affect them all. If that isn't enough some sort of professional help with family therapy may be called for.