Sounds like you have a very good reason to get sober. Those kids need stability and a place to go if their dad is abusive. So you need to keep walking the sober path you are on. If you are potential home for these kids, I suggest you get to AA and get a sponsor. Courts value the program and value a sponsor's opinion sometimes. Did you discuss the abuse with your step daughter? If she were to leave him and move in with you, would that be acceptable? Maybe she needs to know that. Taking on the full financial responsibility is often the biggest barrier to leaving someone. Most of all, if you believe your grandchildren are in danger from this man, then you must report him.
There is not a drug on Earth that can make life meaningful.