Something else I found interesting was JB asking the police if it was possible to tell what order his family had died. A rather curious question, which he said was about the inheritance (shows where his mind was - your entire family has just been wiped out and your main focus is money!)
I've always wondered if he deliberately killed Sheila last just in case it was possible to tell what order they'd died, or if he killed her before June and/or Nevill and was anxious about the true order of deaths being discovered?
As a practical matter there is little way to tell who dies first or last unless there are different causes of death such as smoke inhalation killing someone where there is a fire but others not having smoke in their lungs, that would indicate some were killed before the fire was set.
Even today with all our various tools avaiable the matter of minutes cannot be distinguished and on extremely rare occasions where they can determine someone would have survived longer than another victim it won't necessarily say who was shot first.
With that being said in order for his farce to be true Sheila needed to kill herself last or the others would not have been dead. I doubt he believed police would be able to tell who was shot first though. So he didn't really have to worry about it except he couldn't kill her far in advance of the others as that might be able to be discerned.
The claim he needed to know for inheritence sake is actually BS. Which parent died first matters not. Depending upon their wills it may or may not matter how long passed between their deaths and hers. Most wills have a clause that requires death of a beneficiary to occur 1 month later for the assest to pass under the dead beneficiary's will. If that is not the case with their will then potentially her will if she had one could have dicated what happened to her share. Either way he had to kill his nephews to inherit everything or they would get Sheila's share. That is why he had to do it when all of them were together.
If Sheila was depressed and wanted to kill herself and her kids she could have done so whenever she wanted. The killing of her parents was something Jeremy needed not her. The timing of day when it happened also was suited for Jeremy's purposes.
I think Jeremy was asking police though just to see if they were on to him and so he could see what potentially they had to dispute the notion Sheila killed them all and committed suicide.
I personally suspect he might have brought her in the bedroom and shot all 3 of them and while aiming at others that is how Nevill was able to escape the room. He thought the 2 shots into Nevill's face were sufficient but they were not and then he shot 2 more times as he was moving so hit him in the arm and shoulder only. That could explain his concern. Maybe Nevill was shot the final 4 times after Sheila was already dead.
We will neve rknow for sure though.