Rob seems to be saying Nevill would not do certain things if it was Sheila.
I am agreeing!
A person can only know that they have been shot by physically entering the room. Even if audible shots are heard from an adjacent room or another part of the house. Of course, if such shots were heard, then the worst would have been assumed and it is a horrific scenario. If Nevill was downstairs as some suspect, his way could have been barred, as depicted in the CGI images David produced. He cannot reach the twins to physically check on them because his own daughter is firing a gun at him.