I am sooo sorry to hear you are in such pain over this.
There is NOTHING in the whole world that You can do. You cannot change the way of the world or your Step-dad. If you sit and think, what is taking the blame for something he did going to do. I think it will prolong the fact that he is in his addiction again and he has woken up that sleeping gorilla. Have you ever tried Al-Anon? You can't keep controlling your mom or his mom or anyones mom from finding out. Your intentions are great but they are only hurting and prolonging the truth.
Good luck, hate to see anyone in this pain. I am a recovering addict and I have to see my dad in his addiction still, but there isn't anything I can do. I just stay clean one day at a time and the rest will happen how it is supposed to happen. I have learned I am not god.
thanks for at least reading.