Carol Ann Lee posts a theory about the order of the shootings in her book.
I think you’re right @ToEarlyForDecorations and that order makes sense. Neville was always going to be his biggest threat.
By the number of bullet casings upstairs it’s known that the initial injuries Neville received were inflicted there, The final four fatal shots took place downstairs.
It makes sense that Sheila May have wandered in to her parents bedroom if she heard a scuffle. At this point it’s not known if June had already been shot. However if she had then the first shot which seriously injured Sheila might have caused her to pass out.
Jeremy then goes and kills the children before coming back to discover Sheila still alive at which point he shoots her in a final shot which kills her. He then stages the scene.
According to Julie Mugford he told her a hitman carried out the murders and that Neville had been difficult as he fought back.
I can’t begin to imagine what it must be like to know someone you love did something so awful. She must have been terrified. I have to say though that had that been me then seeing two six year old boys shot dead would have had me going straight to the police after viewing and identifying them. I don’t think she was thinking straight though which perhaps explains it.