Your stepdad crossed an important boundary. The “incest taboo” exists to protect young people and their older relatives from making a psychologically dangerous mistake for them both. For some men, living with an attractive young woman, especially a young woman who is not biologically related to them, is very challenging. Their sense goes out the window when their body responds to the attraction. Some stepdads and even some dads distance themselves from their teenaged daughters because it’s the only way they know to keep the girl and themselves safe. When that happens, it’s often very confusing to a girl who has been daddy’s little girl to have her father start to act cold and aloof. It’s a sad solution for them both.
To your stepdad’s credit, he stopped when you told him to and apologized without prompting from someone else. It sounds like he fully understands that what he did was wrong. He did not make excuses. He did not swear you to secrecy. He is appropriately ashamed of himself. I’m not saying that this makes what he did okay. It’s not. But it does sound like you can repair the relationship if you want to. Up until now he’s been a good dad to you so it’s maybe worth a try.
I do think you should tell your stepdad that he needs to tell your mother what he did. Then the three of you need to have a discussion about how to live together safely. If he is truly ashamed of himself and asks for forgiveness, you – and your mother – could decide to give him a chance. Only the two of you know if he has enough good qualities to balance out a serious slip in judgment.
I wish you well.