I don't think you can blame it on Santino like "he weaken the family". it was a rough time for the Corleones and Sonny did exercise bad leadership but I don't think they could've avoided Sollozzo powerquest.

the way Puzo and then FFC put it, it's like the Corleones are admirable, legitimate businessmen minding their own business when the dark side of the mafia confronts them trying to claim what Vito and his reign gained with their hard work.