Is it OK to have with your stepmom?

This is completely wrong. Both you and she have betrayed your father. Honestly, at this point, he should leave her, and you should not blame him if he's completely disgusted with you. No matter what the age difference, she married him, and thus should have been loyal. You should never have done this, and betrayed your father, and shamed yourself. What you need to do is confess, and accept the heat for what you did. Your father will be devastated. Horrible thing to have done! And, before you try getting out of it to avoid hurting him, think. If she would do this with you, she would with others, and thus can't be trusted.