hey, Hops, When I read this thread something keeps coming to mind. (for what it's worth)
It was something my mentor said about my husband's mother and his brother (and THAT is a spych nightmare) Anyway, what she said was: "they have an agreement".
What she meant was that the mother and the brother, energetically, have an agreement to do whatever little thing they do together, and although my husband may hear from his mother that "she loves him better and is just humoring the brother because of his mental illness" (like that's not the tip off to the weirdness) the truth is: they have their own little thing going on. And her advice to my husband: just do what you need to do and stop concerning yourself with getting approval from those two, because when they are together, nothing else matters but their "agreement". I think that's good stuff for everyone to remember.
So, when your mother asks you to "forgive" your brother....you might consider detaching (actually it seems that is what you did). Her relationship with him has nothing to do with you, or it shouldn't anyway.