Well, in market research we have an interviewing approach called "laddering" which is when a person answers a question like "What do you want from your alcoholic mom" and they say "I just want her not to be an alcoholic anymore," then you would ask "Why?"--because saying she doesn't want her to be an alcoholic is just the tip of the iceberg. So that first answer may be unrealistic, but you/your DH may be able to address some of those deeper things she's starting to share with you.
I remember when my mother told me that she was remarrying when I was 13. She had dumped my dad because of his alcoholism. She had promised me (not too wisely but i understand why she did it) that when "Daddy gets better" she'd remarry him and I could be the bridesmaid.
So, when I collapsed in sobs and sadness after I learned she was marrying a guy that I actually really liked, she asked me why I was upset and I told her that it was because she'd be excommunicated from the church. Of course, that was hogwash--but even I didn't know it. Of course it meant the end of my dreams of my Dad getting better and living happily ever after with us.
I agree that the only useful reason for you to ask her that question is so that you and your DH can understand her better but know that the real answer may be buried within.
As it pertains you playing "telephone" between SD and her mom, I agree with Suki. Let it go. She is lucky to have such a kind, supportive, understanding stepmom. BTW, my stepdad, after I got over crying about it, was the best thing that could have happened to the family.