Maggie/lookout I myself don't think anyone else was involved in the murders at WHF other than one of the occupants in the house that fateful night 
Susan, I have to go with that, too. If Nevill said "SHE has .......", it CAN be ambiguous, "SHE" being either a diminuative for Sheila OR a reference to June/Sheila. It's a HUGE ask in the belief stakes, but it's JUST possible that it was June who cracked first. Having to cater for exta bodies in the house, having to look after/cook for/wash for small children, having to provide ad hoc snacks and meals for the harvesting men, having to cope with her daughter who, for all the help she gave, may just as well not have been there, having to cancel a meeting because she was probably too exhausted/worried to focus, even telling her sister that she was concerned about Sheila, it sounds as if she may have been at her wits' end.