I was attracted to this thread because of the "we" in the title - Yes, i think everyone should let go of the anger, because ultimately it hurts us more than the person the anger is directed at.
I think the point about it being sailing (and not being locked in the cellar) is one that the DB needs to consider. Yes, it was a form of neglect, but perhaps it would help your DB get over his anger if he realises that his upbringing was not that terrible and that there are far far worse things that he could have gone through.
He is only 22 which is terribly young really (OK, i'm only early mid thirties, but i know how immature I was at 22), and probably not quite old enough to fully understand that parents are human too, and fuck up all the time. THe fact that the parents are now contrite and realise that they made a mistake is (or should be) a start to show your DB that it wasn't done through malice, just human crapness, of which we are all guilty.
I think there's also things you can say to your DB to help - I think its OK to point out that his refusal to consider reconciliation is hurting you (because it clearly is, otherwise you wouldn't be trying to help) and putting you in an impossible situation where you feel forced to choose between your sick mother and your beloved brother. Also, and again this is a youth thing, I think, if he leaves it too late to reconcile, he may never forgive himself. Better to attempt a reconciliation and say what he has to say to his parents now, while they're still alive, than wait till they're gone and its too late to get the answers he wants.
Perhaps counselling is in order for him, because I honestly don't think that holding onto bitterness and anger for so long about your childhood is going to anything other than fuck your adulthood as well.
[Background to me: Spent my childhood pursuing my mother's passion, a very privaleged one similar to sailing. I hated it, and having the guts to turn round to my mother at 13 and say "I don't want to do this anymore" was the hardest thing I ever did. However, I never for a moment considered it was neglect. My mother is a sentient human being, with needs of her own. She loved me and thought that she was giving me what I wanted. Perhaps your DParents thought the same - i.e. they thought they were doing the right thing.]