It is generally agreed that the twins were the first to be shot, because Nicholas was still sucking his thumb.
Why would Nicholas be the first, when the biggest threat to any murderer would be Neville
The twins would have been easy to overpower. It makes no sense to kill the kids first.
A deranged Sheila is more likely IMO.
The twins being shot first point to Sheila being responsible.
The bloodstains on Sheila's nightdress show she was sitting up when the fatal shots were fired. Combined with the trajectory of the bullets showing the gun was being held with the rifle to the neck and the stock between her legs. Entirely consistent with suicide
Of course you can argue Jeremy shot the twins first that is plausible, however you will also have to also argue that Jeremy gave Sheila the rifle and ordered her to shoot herself. That don't seem credible at all.
Then you have four elements of the case that Jeremy cannot orchestrate
1. The foster care issue.
2. Her illness was centred on her children
3. Sheila was menstruating during massacre.
4. Sheila's antipsychotic medication was reduced by half shortly before the massacre.
All this combined points to a psychotic episode creating the belief in her disturbed mind that killing her children and then herself, was the most appropriate means by which the problems could be resolved.