TB makes good points. But one thing we should keep in mind is that both Puzo and FFC illustrate the idealic Mafia family where family does really mean family. To be sure, the novel states that succession by a don's son to family leadership was by no means guaranteed, but I don't think the Corleones represent the typical Mafia family in a number of ways. In the real world, it might very well be probable that Sonny would have been challenged by the capos after Vito's death.