The worrying thing is that it is easier to put Sheila into the crime scene than Jeremy. It's also worrying to me that you keep having to change important aspects of your story to answer our doubts. If this is such a sure thing, and you've been on this Forum years and years, then why are we having these problems?
That said, at least you're not abusive like Steve.
To me, the evidence suggests that the killer is firing at Nevill as Nevill is pursuing the killer up the stairs. This could explain the position of some of the casings on the main landing and at the threshold of the master bedroom. This scenario fits Jeremy's claim of a pre-incident call from Nevill.
The killer duly pursues Nevill back down the stairs and there is a struggle in the kitchen. As I've explained, Nevill is more likely to struggle with Sheila than with Jeremy, and that struggle would take place back in the kitchen because Nevill is ahead of Sheila and already on the stairs and Sheila is not necessarily out to kill anybody, as such, so the time-and-motion fits better, whereas Jeremy would simply catch up with Nevill because he simply wants to kill him.
Sheila eventually leaves Nevill for dead, re-loads - not fully, just some more bullets - and returns up the stairs to June and the twins. She may have re-loaded twice more, then washed and then killed herself. I certainly don't pretend any of this is simple, but to me, the above fits the evidence a bit better.
You have totally misunderstood the relationships between the protagonists in this drama. Both children were a worry to both parents, let's admit that. Both children were disappointments. I'm sure in June's mind Sheila was breeding fodder for the son of some landed family in the area. Jeremy was meant to learn the whole aspects of the agricultural business, gain respect therefrom and continue the successful enterprise Nevill had made, some of the son's success rubbing off on the father, who could bask in his accomplishments once he had retired.
Neither lived up to expectations. Sheila had an inferiority complex despite her looks (she never realized how pretty she was until somebody told her), I don't think she ever knew what she wanted out of life, naturally attracted to the quiet, creative, arty types and never materialistic, unlike her brother. As her illness developed she lost even more self-confidence and began to retreat into her own world, mistrustful of those around her but never expressing bitterness towards others, rather craving acceptance.
Jeremy had tried for years to emulate his father, an impossible feat for most men, and when he gave up attempting to please he was bent on humiliation, hence the final calamity. He had moved his father to tears on so many occasions, as evidenced by close friend John Seward, that it's impossible to envisage any situation, even an emergency, that Nevill would request the help of his son.