So just to clarify, can you confirm that you don't think he A) Put the rifle down on the bed, picked up a sleeping/dozy Sheila, carried her through to the master bedroom, put her down in a seated position on the floor and ordered her not to move, whilst he went back to get the rifle to kill her with, or B) Stuck the rifle between his legs, picked up a sleeping/dozy Sheila and waddled with her into the master bedroom where he put her down and shot her, or C) He woke/roused a sleeping/dozy Sheila, gave he the rifle to hold for him whilst he picked her up and carried her to the master bedroom where he put her down and shot her.
Everyone is assuming Sheila woke up. There is no reason why she would wake. Thread already created. The twins didn't wake.
If she didn't wake, she was either lead or carried away from her bed. She had to be as she was the one being framed.
Going back for the rifle ? It would be next to him and Sheila on Neville's bed.