You don't need a whack on the head, you need (((((hugs)))))
During the active stage of my daughter's addiction, another young woman spilled her guts to me about her family situation not knowing about my daughter. Yours and her words are almost identical.
It was because of her that I stopped talking to my son all the time about his sister unless he asked, and not so surprisingly he didn't ask very often. When he did, my responses were "its a good day" or "same old crap" and not much else. I also marched myself into therapy.
What I had been doing caused a divide worse than what existed between them. They have healed and I'm sure some of it's because I finally shut up and got out of the way.
I hope your mom finds a meeting or a therapist and I'm saying prayers for your serenity.