Hi justwanttohelp, is the news about your SM stirring up the feelings you had when you realised your mother was an alcoholic? Because although you love both your Moms, it's not your battle, or even your business. Yes you are shocked, but this has been going on for years without your knowledge, and may never change. Is there a particular action or health crisis which has brought this out in the open?
My own parents went through a stage where they were drinking inappropriately, and I was devastated, so I may have an idea of how you're feeling.
Now that you've found out about your SM, you're not required to take action, even if you do have strong feelings. Any movement towards recovery will have to come from her and her alone. Nobody can force an alcoholic to be sober. In time there may be some ways to help her, or support your father, and I'm sure you'll be there.
As for what to say to her; I think NWGRITs nailed it.