Myall's WS states Bamber told him he had loaded the rifle with 10 bullets. If correct, not possible he could shoot the twins first. Once each.
It is possible Sheila was shot first where she slept. Once. The remaining 9 bullets fired into June and Nevill.
The twins remained sleeping behind thick doors & walls. As the evidence shows.
After the kitchen fight & all other bullets had been despatched, Sheila was carried a few yards to her final resting place. To give the impression she had been out of bed. This is when Bamber noticed she was still breathing.
Sheila's bed sheets removed & washed along with his massacre clothes. New bed sheets put on the bed