Sheila would definitely have shot her mother,no doubt about that, which is evident by the sheer frenzy of the shooter,but it's what followed that's the difficult bit,or how it all began when they were all in their bedrooms,presumably settling in for the night.
Why did Neville move out of the bedroom when his wife was being murdered ? Why did any of them leave the boys ? Unless they thought they wouldn't be shot as well,I don't know. Who but a maniac would have killed the children ?
No,I don't believe Sheila shot herself----------not with a rifle anyway,it's impossible.
Do we know if Neville's gun/rifle was under the bed ? If so,for what reason ?