Oh, do come on. Jeremy picked her up and walked her to or from her own bedroom when she was half dead? This simply isn't good enough.
Sufficiently under control? That is ghastly. Just how crackers was she?
Mmmm... I can see this being more difficult to convince you than I thought!
There are different opinions as to the severity of her physical and mental condition. Haloperidol has sedative-like effects. According the CAL's interview with Sheila's closest friend, Tora Tomkinson, Sheila was incapable of getting up from a seated position without help only a week or so before the murders. On the night in question she sounded vacant and vague ("like a zombie") on the phone to June's sister. Jeremy stated that she never participated in the kitchen conversation that night concerning the future of her twins.
My own view is that she was woken up by the noise of the fracas in the main bedroom and on the stairs, and the struggle between Jeremy and his dad in the kitchen; saw the body of June lying on the floor, then tried to hide in the Box Room adjacent to her parents' bedroom, or crouched down so as not to be seen at Nevill's side of the bed when she heard her brother come back upstairs.