Hi, gemini.
Welcome.
I think your ex’s sibs don’t want to talk about him with his mom because they are, as you say, over it.
One could also say they are defining their boundaries with their mom and ex.
Which is a great idea, btw.
How to that?. Remember that no one causes the addiction, no one can control it, and no one can cure it.
Your ex’s actions are his decisions, his choices.
I don’t know what his mom wants to talk about, but I’m guessing it’s some type of justifying why he is the way he is.
Just keep pulling the discussion back to his choices, his actions.
Over time, you will be able to pull away.
It’s a process.
Good luck.