I think the book fleshes out that Sonny was ruled out because he has a wife and kids unlike Michael then but now if he is in the same boat No doubt dilemma!
In the novel, Sonny says (of killing Sol and Mac), "I could do it." Tom dismisses this immediately. "Solozzo wouldn't let you near him if he had 10 police captains. Besides, as the acting head of the family, you can't be risked."
