Vendettas spanning generations are because your social status require it, not because it is universal emotional reaction to someone being killed three generations ago. That is about mafia dominance struggle.
Moreover, other phenomenon in low legal justice areas is that if you or your family is not powerful enough, you suck it up and shut up when someone more powerful kill your relative. Or even when equal does that, unless you are connected to one of mafias. The average person when they have relative killed in drunken auto accident wont kill in revenge.
Even criminal groups more often kill for profit or to get rid or witness then pure vengeance.