It must have been hard for Jeremy, knowing June was giving her money to the church. This could have carried on for another 20 years. This together with the twins being given a larger slice of her will, would have been very frustrating for him.
I have the following observation to invite anybody or everybody to consider...
Killing your entire family, or lets say three generations of the same extended family in order to lay your hands on the entire estates of your parents, or adoptive parents, or whatever, would require more planning than what is suggested that Jeremy is responsible for. Since, it would be necessary for the killer (in this case Jeremy) to kill the five victims in such a wwway that it becam obvious to the police which victim had died first, then next, and then next and next, and so on...
It would require the killer to stage the death scenes in such a way that the police could tell, he had been shot first, to last, in order to guide the inheritance into the killers hands - no such preparation appears to have been undertaken by whoever shot and killed four of the victims, other then the police staging Sheila's death scene on the main bedroom floor in possession of the gun after 9.13am, on 7th Auguist 1985...