If Jeremy committed the crime I speculate this. The Twins most probably died first, one still had his thumb in his mouth indicating he was asleep and by all reports said they probably died in their sleep.
Its extremely unlikely they would have slept thought the massacre had they not been first.
I have always thought that Jeremy would have shot them both first point blank range one shot each to the head, Then once everyone was eliminated he would have reloaded the gun and fired more shots to make it look more like the work of a deranged mind than that of a calculated killer.
Does that make sense?
It doesn't make sense for several reasons.
1) The experts say the shots were fired in succession at the boys. The nature of the wounds and the tight grouping of the shots resulted in the experts saying they were fired in succession
2) Children generally sleep the soundest and are the least likely to wake up/get up if they wake up.
He knew his mother was the least sound sleeper int he house so the most likely to wake up who in turn would wake up Nevill. The parents confronting him is a far greater threat than the kids waking up so the parents would be the logical first target.
3) Had the children been shot first with 1 round each that leaves 8-9 rounds in the gun (depending upon whether it originally had 10 or 11). No less than 10 shots were fired in the master bedroom in the initial exchange with the parents (4 into Nevill and 6 into June). If the boys were shot first there would not be enough bullets left in the gun for 10 shots to have been fired.
The only wound suffered in the upstairs bedroom that could potentially have been delivered later on was June's shot between the eyes. The other shots were all delivered while she was in bed or in the process of getting up so had to all be the same time. Back spatter on the door among other things indicates she was lying on the floor when the shot between her eyes was fired. It is the only round that could have been delivered anytime because she was already dead lying down when it happened and thus didn;t have to happen immediately.
If the gun had 11 rounds at the outset and 2 had been use don the boys how could 10 have been left to use in the master bedroom? Only 9 would have been left and only 9 shots woudl have been fired but at minimum 10 were fired so this contradicts your theory.
Realistically, if the gun had 11 rounds at the outset then the shot between June's eye was delivered right away and thus all 11 rounds in the master bedroom were fired the same time. If the gun had 10 rounds at the outset that means Jeremy came back from the kitchen after killing Nevill with 11 rounds int he gun and that he shot June at that point just to make sure she was dead after seeing what it took to make sure Nevill died he didn't want to take any chances. He also wouldn't want to take any chances with the boys which explains why he fired several shots into them. Nevill being able to put up the struggle he did and June being able to get out of bed didn't exactly instill confidence in his weapon.
It is obvious to me that the boys slept through everything that had happened and were killed after June and Nevill. If Sheila was up by the time Jeremy went back upstairs then he would have dealt with her first otherwise taken care of the twins first. We don't have enough information to know whether Sheila was up already at that point or not so have no good way to assess whether she was killed before them or after.