The second to last point in my thread point answers this.
Bamber had lots of options with Sheila. Physical force, persuasion, coercion, or a combination of all three. Or shooting her while she slept. He only had to move her a few feet.
There is always the slim chance she did wake, didn't go to the twins, and then somehow avoided Junes blood. Bamber would then just shoot a terrified Sheila.
These options only apply if Sheila was in her own room. You have said, innumerable times, that Sheila wouldn't have seen June because it was dark. The light switches in old houses are generally just inside the door, to the right of where it opens. As June fell by an open door, it would suggest that, at some point, the door had been opened so all Sheila would have had to do was put her hand out. She knew the house. If Sheila walked from her room to the master bedroom, I doubt she'd have done it in the dark. I imagine she'd have put lights on if Jeremy hadn't already put them on. I can't think how, with lights on, it would have been possible, if June was already dead, for her NOT to have seen June who was almost in the door way.