Hi Jan. If she shot the twins first then she would have had to reload. 9 shots? leaving a possible 2 left if the magazine was full. She had no pockets and I think it would be unlikely that she carried them in her spare hand.
Neville could have been woken at that point though and had followed Sheila back down stairs completely unaware that the children had been shot. He could have had an argument with Sheila who had the rifle already in her hands and was refusing to give that rifle up. He then called Jeremy for help this angered her like it did when Freddie phoned for help and she cut him off and ran upstairs with the rifle and he raced after her and just plonked the phone down leaving it engaged. She could have then fired at her mother and then at her father as he entered the bedroom. NB then turned round to get downstairs and she raced past him to reload, but before she could fully load they had a scuffle at the sink....Just my theories Jan......could be totally wrong...But it might be better than this one.
Jeremy put on his wetsuit and peddled through the back wacks to WHF. He then entered the house via the downstairs bathroom window. He collected the rifle with silencer from the den or a hidden place somewhere else in the house and placed the fully loaded magazine into it, or it could have already been in. He crept upstairs entered the children's room and fired 9 shots. He then crept downstairs to reload thus giving him 11 more bullets to fire. He then entered the main bedroom and shot his mother several times, then shot his father as he managed to get out of bed and into a standing position. Jeremy then managed to fire two shots to Neville's jaw. (important point coming up) NB then had to get passed Jeremy so that he could shot him from the back for the shots to his shoulder and arm were fired when NB had his back to the shooter. JB then rushed past him shoving NB towards the wall where there were blood trace evidence. He got to the kitchen NB tried to get the rifle of him they had this almighty fight at the kitchen sink. NB then had 4 more shoots and was beaten over the head with the rifle. JB then went back upstairs woke Sheila up and led her to the main bedroom she was told to lie down and he shot her twice. He then showered in his wetsuit, phoned home, swapped the phones placed the silencer back into the cupboard, went back into the kitchen bolted the kitchen door then climbed out of the kitchen window because he knew how to lock it. He ten peddled home via the sea wall and got out of the suit hung it up. The invented a call from his father.
Which sounds the most far fetched?
Sorry about the length of this post Jan....I'm not a chatter box in real life.

So a fully fit Neville was unable to get the rifle off Sheila ? So Jeremy was phoned, rather than June.
Neville and June did not hear her shoot the twins eight times beforehand. Although Jeremy said the rifle did not have a silencer on.
If Sheila then shot Neville upstairs with the remaining bullets, after the twins had been shot, he would have received only three bullets. June continuing to sleep through it all.
The prosecution's version of events is much more straight forward and plausible. Everything fits.
I agree he did not shoot the twins first, he went straight to the main bedroom emptying the rifle into June & Neville in/by the bed. The twins sleeping through it all as the rifle had the silencer on.