OP I haven't read it all so I don't know how the thread turns out, but I have a family who want everyone to 'forgive and forget' at the expense of ANYTHING ever being sodding sorted out
So when I had an astonishing attack from a sister (long time ago now) who laid into me with no provocation about something I'd not even done - called me all sorts of names under the sun, told me no one in the family liked me, that I upset everyone and thought I was so superior.... I needed lots of support, frankly. I'd always believed her in the past but this was the first time I realised she was putting me down because she felt awful about herself. I needed siblings and parents to reassure me that no, I wasn't any of those thing she'd said and also no, I hadn't done any of those things she'd said, and above all, yes, she was unhappy but it wasn't anything to do with me.
What I got was "we don't fight in this family" and "I hate that everyone is so unhappy" and "can't you just let it go?"
NO, I bloody couldn't. I was deeply, deeply hurt, I'd been wronged and I wanted my feelings to be taken into account
Asking your brother to forgive and forget (in the long term) is reasonable. But to ask him to do it instead of to acknowledge how he feels and accept that he has every damn right to feel this way, because he has been wronged and betrayed and deeply hurt by what his sister did to him ... you asking him to sweep his feelings under the carpet and pretend everything is fine will not make this problem go away.
Of course you wish everything was the way it was before. I'm sure everyone does, at some level. But telling him he can't feel what he feels and he's not justified in feeling what he feels will NOT make it better. It will make him feel that no one supports him even though he's the injured party.
Please don't let your desire for 'everything to be alright' actually stop the healing process. And good luck.