In short, I believe that Sheila in a worsened state of her illness first shot her sons, thus waking up her parents. Maybe Nevill ran downstairs immediately to call for help, then upstairs again to try to prevent more damage. In the meantime, June was shot (maybe just once). Nevill was shot once in the bedroom, then, at gunpoint, led downstairs (no more rounds in the rifle, Nevill couldn't know that).
The fight, possible reloading or another gun was ready downstairs. He was shot. Upstairs, June had tried to get to the twins, but heard the commotion downstairs and turned around, maybe she collapsed at the door then. Sheila upstairs again, shooting June (still alive) again a few times.
Sheila cleaned up (her hair in the crime scene photos looks like freshly washed hair that hasn't been combed.) Maybe she ate. She wore the socks and went upstairs to her mother at some point (perhaps hours later) and shot herself after removing the socks.
Maybe this is totally out there, but this is roughly how I imagine events that night.
I have to apologize to Sheila if I am wrong. It is a horrible thing to say about someone who is possibly a victim of a murder herself. Sorry Sheila, if I am wrong.