I realize all of that about Sonny not being able to stay couped up, about his needing to see Lucy, etc.

But the real point here is that he was negligent in that he would put his sister in such a vulnerable position during a time of war. Think about it, Sonny even wants to send Michael, a 'civilian' at the time, over to Luca's place and I believe that it is Tom who suggests that they should keep Mikey out of it.