You have just killed your entire family. Stage the killing of your sister as a suicide, setting out to pin the murders on her - but you shoot her twice, oh well, never mind, the stupid coppers will buy that, you tell yourself. You then unscrew the silencer you used, hide it in the back of a cupboard, blood, paint and hair on it, but heck, the cops ARE stupid, you think to yourself.
Later when you meet with the cops outside the house, you tell them all about how you are going to buy a Porche and that you don´t like your sister, whom you just killed. You also think it is a great idea to accuse the police of shooting your family once they found the dead bodies.
It has a ring of truth to it, RIGHT?! 
It makes alot more sense than SHeila doing the impossible and killing eveyrone else and herself without getting their blood on her or any GSR, after she was dead moving her body flat, placing the bible in a pool of her wet blood and putting the moderator in the closet, Julie making up eveyrthing, Nevill calling his son instead of arming himself, disarming Sheila with his bare hands, or calling 999, Sheila knowing how to chamber a round though she never used the gun before nor did they ever use other semi-autos around her so she had no idea a round even needed to be chambered not to mention June staying in bed through everything and Nevill marched upstairs so they coudl be shot together plus Sheila deciding to get 5-7 rounds from the closet instead of using all 25 rounds from the pile of bullets in the kitchen.
Anyone suggesting such is likely is living in a fantasy world.
This is before even looking at Jeremy's actions. Anyone suggesting Jeremy's alleged actions make sense if he really had received a call from Nevill likewise are living in a fantasy world.
Receiving a call from Nevill then instead of calling 999 or rushing there to calling Julie and eventually look up the number of police stations, to be required to go to the scene so drive slow making sure police arrive first then for him to lie to police saying Sheila fired all weapons and calmly chatting with them instead of trying to look in the windows or go in the house like he had the ability to do (though at the time he lied and said there was no way in except breaking down the door), calling Julie at 6Am to tell her not to go to work because she would need to tell police about the call he received from Nevill... makes no sense at all unless he killed his family.