I have!
OK, I will give in to you request for a "scenario". It is bound to be flawed, and I don´t really like doing this, but here goes.
I believe the twins were shot first. In a way I think they were the main target; the twins and Sheila herself. Sheila had some very unsettling thoughts about them: they were the Devil´s children, and she was afraid they were capable of raping and killing her (Sheila´s own words). Also keeping in mind June´s words to Sheila: You are the Devil´s child, in her twisted mind Sheila could have done this as a revenge to/request from her mother.
The shots woke her parents up, or Sheila came to the bedroom with the rifle shouting at them, I don´t know. Her parents in their wildest dreams would not imagine she had shot the twins or would shoot any of them - if they had heard the shots, they would have thought she just shot into walls, floor or ceiling in the corridor. Perhaps Neville managed to calm her down momentarily and thought it would be safe to run downstairs to alert Jeremy (and the police??). Then all hell broke lose in the bedroom, perhaps shouting, perhaps Sheila shot June who was still in bed, petrified. Neville heard this.
Enter Neville in the bedroom, got shot four times, stumbled down the stairs, got shot again on the way down. Sheila after him. The fight took place - Neville, severely injured, would not have been hard for Sheila to overpower, I think he received some of the beating sitting down actually. Why the fight took place, I don´t know, probably because she needed to reloand the rifle, maybe paired with the fact that Sheila felt especially betrayed by Neville who had hitherto been on her side, but recently had agreed that it would be a good idea to put the twins in foster care. Who knows.
Sheila goes upstairs, shoots June between the eyes as a final act of revenge and contempt.
Sheila might not have gone to bed at all that night (stomach contents support this - the look of her bed supports this) and would have still be wearing her day clothes during the killings. She might have have taken off those clothes, put them in soak (sounds odd, but the atmosphere about Sheila in death is "clean", she looks very, very clean. Perhaps this whole thing was a catharsis for her), taken a bath and put on her night gown, prepering herself for her suicide, reading the Bible, then shooting herself close to her mother for whom she had done all this: ridding the world of the "Devil´s children": Herself and the twins. "Ha, mother, see what I did, I did it FOR YOU!"
This makes sense to me, there is a logic in it, if I may say so myself! Sheila was a woman who had lost EVERYTHING. Her career, all petty jobs, her husband, her sanity and was now about to lose her children, whom she had already lost partially.
My queries on this -
We only have Jeremys word on the potential fostering.
The children were shot 8 times. Without a silencer would she have the time to do this before being restrained ?
Neville & June calming her down will result in the gun being taken off her. Neville & June will now be aware of the twins death & will have the gun.
June would have been sent downstairs to make the phone calls. Neville is stronger & gets on better with Sheila.
Neville if downstairs did not need to ring Jeremy at 3am. The damage had been done & things were under control. The police were coming over.
If Neville heard shots from upstairs, he only heard two. The gun only had ten bullets, eight which had been used on the twins. He would have restrained her upstairs. June would have only been shot twice.
If everyone was shot & Sheila changed. Would the police not see bloodied clothes in a sink ?