I keep thinking about this more. Again just my two cents, you do what feels right to you. But way back when I was dealing with this I felt so isolated. She was the only one that knew for quite a while.
if you are wanting to do some thing helpful for your daughter-in-law that could also help your son, encouraging her to learn about alcoholism and telling her that what she's dealing with is not normal and that she should not be doing it alone and isolated, repeating that to her whenever she calls to vent about your son. That could help her which in turn could help your son and give you some emotional relief. Because those two in that house, if could go on for years if she is codependent and not working on herself and enabling his behavior.
I don't know, I am rambling. I am just thinking about things from your perspective and what helped me when I was in that situation.