Back to Sheila's wounds, and the position of her body. I gather that it was the upper wound which killed her, which I find strange as there appears to be smudges of blood around it, as if she had clutched at her throat.
I did notice that the door behind her into the box room was open in the photos, so it's possible that she went into her parents' bedroom that way. That would suggest that she'd been in the twins' room prior to that as I think there was a door to the box room leading from their room. That doesn't really explain much, but I've always wondered why she was shot in that particular place, whether Jeremy shot her or she shot herself. It's odd though - wouldn't Jeremy have preferred to shoot her in a place which wasn't visible from the front of the house - in her own room, for example?