I do agree but then I have always found it hard to believe that he could have crawled through the window, killed all five people even if Nevill was in his expected place he still got away and ran downstairs.... the risk of taking on Nevill was so massive..... fighting over the rifle with Nevill, bashing poor him with the rifle without a mark on him and dashing back over the fields etc. and the rest is history.... it's very difficult to take on board and that is the problem for me.

There is always the feeling if he was involved he wasn't alone but I cannot believe it was Sheila because unless psychotic she just wouldn't have done it. She wouldn't have calmly planned it because it would have been crazy for her to believe she could overcome Nevill and she would never have killed her children in her right mind......

Siblings that kill their parents come in two varieties:
1) Siblings who have a joint hate for their parents and are aware of their joint hatred
2) One sibling has significant away over the other siblings and calls the shots and the others simply do what they are told. Most often this is true with an idolized older brother but there are other paradigms.
What you find is that the siblings are closer to one another than the parents. Sheila and Jeremy were not close, they do not fit any of the patterns at all. So as you have observed there is simply no realistic way to say Sheila was involved.
But Jeremy didn't need anyone's help anyway. He picked the middle of the night so that he would not have to confront all of the victims the same time in the same place. He didn't need anyone to sucker victims to their death, he went to them. He did't need an inside man to provide the layout of the house etc he had the requisite knowledge and access. The only aid he could have used was with respect to an an alibi and he used Julie for that to the extent he could. Calling her to tell her Nevill phoned was supposed to be proof Nevill called him. Julie chose not to frame the call in such manner because then police might think she understood he was using her to support his alibi but that is what it was for though he called to discuss his excitement as well. He could not contain his excitement and that could be what delayed him calling police he needed time to calm down and prepare to pretend to be scared and upset- that and he needed to work out exactly what he wanted to say to them.