Susan there is virtually no possibility of Sheila waking.
It depends on how light of a sleeper she was and how much noise was being made. It is possible she woke up at some point of her own volition as opposed to Jeremy waking her. Only Jeremy knows whether it happened.
What is obvious is that if she did wake up and leave her room on her own volition as opposed to at gunpoint she wasn't out of her room very long before Jeremy showed up because she would have gone to check on her kids and tried to hide them or get them out of the house. The fact she didn't do this means either she never left her room until Jeremy forced her at gunpoint or took a while to wake and leave her room, saw her mother's body then Jeremy was returning up the stairs with the loaded weapon and she tried to run away by going through the boxroom but her stopped her before she made it.
It is more likely that he woke her up and marched her at gunpoint than her leaving of her own volition but suggesting it is virtually impossible for her to have woken up is not true. The odds are simply more likely than not, not beyond a reasonable doubt as you suggest.