You've got it in one, jacknch.
That's the conclusion most of the original members here have reached - although there are differing views as to how Sheila Caffell finished up where she did. Your theory could be right, but an alternative one is that if she was the last to be woken by the racket in the bedroom, on the stairs or in the kitchen, she may have opened her own bedroom door to see her mother lying on the floor, immediately went to check if Nicholas and Daniel were safe, and then heard the intruder return upstairs. She might have assumed it was a burglar rather than her brother, so tried to conceal herself in the Box Room between the twins' and parents' bedrooms, or entered the latter from there and tried to hide by crouching at the side of the double bed.
If Sheila was dopey/drowsy from the effects of haloperidol, then she may have been easy to control and lead into the master bedroom, but why do that when it was simpler to shoot her in her own bedroom ? I could be wrong, but that's why I prefer the hide-and-seek theory.
Blood spots were found on the landing carpet so it's possible those two tiny ones (if that's what they are) on Sheila's sole were picked up there, but to inflate it into new proof that she killed her family is laughable.
Whichever route Sheila used the end result was the same - murder not suicide.
