I cut off contact from my father, but I left it at if he ever got sober he could call me again. It was very freeing from guilt to put the owness back on him about making contact. He did try to contact me once when he was drunk, I just reminded him not to call me until he was sober and hung up on him. I was an adult, I didn't need to deal with his drunken crazy stuff.
My brother was still in contact with Dad, but I asked him not to talk to me about him and not to talk to him about me. He asked me what to tell him when he asked about me. I told him, just say that any time you want to know, just sober up and call him.
It sounds like your conditions for future contact need to go way beyond simply being sober. Maybe something along the lines of when Mom is sober and stepfather is willing to ask for proper forgiveness, not just ask you to bury the pain of the past. Proper forgiveness to me would be admitting publically what he did. Promising to never do such a thing to another person (are your nieces and nephews safe? you may want to alert the police) and offering to pay the penalty for his crime (confess to the police, or pay for your therapy, whatever it takes).