I am close with my ex mother-in-law, I used to vent to her only because her presence was a lot of the reason I was able to see I wasn't crazy about my then husband's drinking. He always made me feel crazy, but then he would behave like that with her around and I Would get validation from her saying he shouldn't be like that and that I wasn't crazy. You are in such a tough situation, I have majorly backed off sharing information with my ex mother-in-law now that we are not married, I feel for you and your daughter-in-law. Hopefully she is seeking help in counseling herself.
I did not get a lot out of Al-Anon, so I cannot be of much help there. But this online support community changed my life. You can never post too much, and you can vent as much as you want here. I did not feel that same support in Al-Anon, it was too regimented for me and I did not like that and encouraged people to stay in toxic situations. There is just so much more to life, and everyone deserves happiness. Just my two cents. Keep posting here, don't ever feel bad about it, posting here it's just part of the process of your own healing.
you did say some thing though, I think it would be understandable for you to say to your daughter-in-law that you think she needs to get unbiased help from a therapist, someone who is knowledgeable about alcoholism and tell her that you're struggling hearing these issues with your son but being unable to help her. Her venting to you is horrible for your own mental health, which is something I realized I was doing to my ex mother-in-law and I stopped. It was hurting her, and it was hurting me. And no one was getting actual help. My ex mother-in-law would often have conversations with my then husband, but it never did anything. You guys are just in a toxic cycle that will not fix itself until someone does something different. Does that make sense? I used to envision myself on a carousel, going around and around and around And I realize the only way to change was to get off the carousel.