Welcome Strong....
Last week I was talking to a woman that used to head up the Domestic Violence Center in my city. She said that many times what finally gives a woman the courage to leave is her children and the promises she makes to them to keep them safe, healthy, and happy. We all find resources that strengthen us and doing it for your kids is a great resource.
Maybe you are hoping that you can preserve something for your kids that you did not have. But......it sounds like that is not going to work out with your husband at this time. Someday, if and when he is clean and sober it is his relationship to repair with his kids. You can't hang through the "bad" and think that it's for the right reasons. Protecting kids from turmoil/addiction/etc. is key. As you know, growing up in an dysfunctional family only creates problems later in life. I think it is better to expose children to no father than an addicted father. You can remain open to their having a relationship with him when he has demonstrated that he is sober (say for at least a year?).
I recently moved out of my home with my husband and I found that letting people know what I was dealing with was key. It kept me accountable. I felt a whole lot of shame in admitting what I had put my kids and myself through. The only thing that could make it worse though would have been to continue to stay. Your stepfather and mother would understand what you've dealing with if you told them. I'm sure that they love you and want what is best for you. Telling them what is going on would shed light on a nasty secret but that might be exactly what you need to get on the right track.
I know that I could not have done it alone. It is humbling to tell on yourself and ask for help - but it's worth it.
Sending you warm thoughts....