Private
Do you want to know for curiousity's sake? If you find out that there was foul play between them, would you seek to try and right the wrong? Or would you leave the past in the past?
I don't think there's anything wrong with that. I think it would be wrong of you to ask or expect anything else, honestly.
He means more to me, too. I've probably spent more time with him in the past year than I ever did with our father growing up. I've asked him a few times, so I don't think that he'll answer me truthfully as long as the good doctor and I are at odds.
It's hard to leave something behind when you live by the motto "No man left behind". But whether you share a burden or leave it behind, the point is to not let what happened in the past control your future.