In the book Sonny inherted/rose to power ( I guess a bad choice of words but they mean the same) when Vito was shot and out of action. It's part of the story that was never told in the movie and even Puzo wanted to explore it more. I think with a little creativity that story becomes interesting. It doesn't have to reflect his skills as a don but he had a unique talent for street warfare, I think the book put it that he was like a young Napoleon. You could contrast that with Vincent having that same unique flare and qick temper like his father but regulated by cold cunning like Micheal and reason like Vito.