I agree SC
Vito knew Fredo didn't have smarts to ever be Don; he knew Sonny's temper was not a good trait for a Don. Michael and Tom knew this as well. BUT, don't you think that even as wise as Vito was and as smart and cunning as Michael was, the last thing that either would think is that Fredo would be disloyal to his own blood. In Michael's cold/chilly way, Fredo probably
really did break his heart.

TIS