This is a very difficult situation for you and him, your daughter and grandchild. His relapse (which hasnt taken place) is not cause for you to treat him badly. Operating off of fear truly never gets us anywhere. They are married whether you like it or not. You dont have to like it. I think that you must respect their decisions and accept it.
Have you ever struggled with anything in your life? Addiction in some form? Are you willing to look at addiction and recovery with a open mind?