I've always read on here that it's assumed the twins were killed first. Surely there would have been a lot of noise which would have woken them if that's not the case.
That assumption is not from the prosecution experts it is mainly by Jeremy supporters.
There is no way that Jeremy risked waking up his parents by shooting the boys first. Moreover had he done so then he would have had to fight his parents with far less rounds than he did.
He fired at least 10 and possibly 11 rounds in his first attack. That attack was in the master bedroom. June was shot 6-7 times in this attack and Nevill 4 times. I say 6-7 because for sure 4 shots were fired into June while she was in bed. 2 more definitely as she was in the process of getting out of bed so could have been out of the bed. These 6 shots were all fired to her right side. The final shot was between her eyes. This shot was not fired until after she walked around the bed to Nevill's side and then back towards the door collapsing. The shot was fired between her eyes to make sure she was dead. Logic suggests after Jeremy was gone is when she walked around the bed and back then collapsed and that he fired this shot when he returned from the kitchen but it is not impossible for him to have done it in the initial attack.
The gun was empty and that was why thing progressed to the kitchen. Either Jeremy ran down to get more ammo or Nevill ran to try to get a shotgun to confront Jeremy with and the pursuer caught the one fleeing in the kitchen then the struggle took place. Jeremy needed to put the gun down to reload the magazine so could not reload until Nevill was beaten unconscious. He reloaded then shot Nevill 4 times in the head killing him.
If he loaded only 4 rounds the gun was empty and then he fully reloaded it before going upstairs. If he reloaded more then that meant 6 shots in the gun (1 in the chamber, 5 in the magazine). Rather than going upstairs with only 6 shots in the gun he reloaded the magazine fully so had 11 rounds.
If when he went upstairs Sheila was in the hall looking at her dead other then he ran after her and had to kill her before the boys. Otherwise he killed the boys first (8 shots), fired one in between June's eyes to make sure she was dead, leaving 2 for Sheila. (naturally if he went upstairs with 11 during the initial attack then it means he only went upstairs with 10 after the kitchen episode)
Jeremy feared them finding out the twins were killed after Sheila so he would have good reason to kill her last and it seems he did because even though drawback remains after several shots the blood right in the opening would likely have not been there if 8 shots had been fired subsequently.
So in all likelihood he killed the boys 3rd and 4th and then Sheila last.