I really don't think that Neville went to bed,or even got as far as the bedroom itself. He was late finishing in the fields,then he took the dogs out,came back,probably had a drink of tea and a smoke,then showered and put his pyjamas on,so it would have been quite late/early morning when Sheila kicked off,if she hadn't already killed her mother and the twins.
It's the way I see it. The three upstairs could quite possibly have been killed while Neville was out with the dogs,so that Sheila only had to contend with her father,and with a few rapid shots would have put him out of action against trying to fight back.