@DSC:
I'm not sure it's random. It's a big plot point in a play I was forced to do as a kid in high school. Two kids meet and slice their palms then put the wounds together and become blood brothers… while not knowing they really are brothers and one was adopted.
I like One Piece more. xD
I don't think Sabo would do 'something unforgettable' to Ace and Luffy unless he knew it was somehow for their own good. He's risked his life for them on a number of occassions.
And as someone has posted repeatedly, once Luffy decides you are an alright person, he just will not accept betrayal as a reason not to support you.