Ah, the famous Santino Corleone temper!! However, I think that Sonny knew his role as head of the family was only because Vito was injured. And Vito would have been angry at some of the things Sonny had done, like killing the Tattaglia son. He would never have let Sonny remain in power. And the truth is, Fredo was never jealous of Sonny. Sonny was older, so Fredo had no reason to feel that he was bypassed for the role of Don. He never would've been a threat to his older brother.
However, I think that Sonny knew his role as head of the family was only because Vito was injured. And Vito would have been angry at some of the things Sonny had done, like killing the Tattaglia son. He would never have let Sonny remain in power. And the truth is, Fredo was never jealous of Sonny. Sonny was older, so Fredo had no reason to feel that he was bypassed for the role of Don. He never would've been a threat to his older brother.