Yes, I believe Sonny's weakness was his children. It's very hard to blame him for that, but I also think it helped Michael that he didn't take the stand against any made members. On the other hand, Sonny would have gladly killed other peoples kids for doing what Michael did. The crazy thing is that Sonny was actually one of the first people who lobbied for the execution of John Gotti Jr. when it came out that he had proffered. In the end, he was just a murderer and a hypocrite.