Hi ~ Alot has to do with the REASON your brother acted the way he did. Why did he send this e=mail. It doesn't sound like he was abused in any way. So what's the matter with him, anyway?

Is he jealous, or what?
Since this was mailed to your mother, I really don't think you'd better get involved. Your therapist was right. Your parents CAN fight their own battles, so to speak. And obviously this e-mail was written in anger, so whatever he said about you was probably said in anger too, and he MAY not have meant it. Who knows. So to address it now would be to just push him further away. I'd leave it alone.
Who knows what time will do. Perhaps later, he'll have a change of heart. Time DOES heal. But in the meantime, try to help your Mom and Dad to heal themselves and to stay positive. They can still keep him in their hearts. God bless and please take care. Hugs, Lee