Hi Wmm,
I understand trying to watch interactions between family members. Gently, I say to you that your Bro is an adult and you can't control any interactions between him and your mother. I know you want to rescue, but maybe you can't and you have to be gentle with yourself and be able to forgive yourself .
It appears in August you were given some advice on triangulation and boundaries . I will add the link to boundaries here that may help:
https://bpdfamily.com/content/setting-boundariesInteracting with our family members, especially when one is ill is a minefield.