My mom has a new partner after my father and i was wondering if it is a sin not to obey your step father?
Tim, you seem very unsure about most everything based on your posts. Make an appointment to talk to your priest about your concerns.
As with all things it will depend what he is requesting from you. Is it morally wrong? What is the motive for not obeying? If it is because you are angry then perhaps he is not the issue but look inward. I could not presume to know if this is true, but if this is the case and it’s an issue of resentment I have always found the truth to be emancipating.
In addition to the above, are you living in a home and consuming resources provided by him?
It depends what your step father asks you to do. If he asks you to clean your room, then you possibly have to listen to him. If he asks you to go steal a candy bar, then you don’t have to listen to him.