If Sheila had become increasingly obsessed by religion and had grown up in a religious household (presumably this involved going to church) - why wouldn't Sheila herself have some knowledge of the bible? Because of her illness she had talked of her children being the devil's children - the Transfiguration is about being reborn (after death) and cleansed by god. Had the family been to church the previous Sunday? And had the Transfiguration been part of the service? As a Christian feast - why wouldn't it be? My brother was a paranoid schizophrenic (sadly also committed suicide) and I know how their mind twists things, it need only have been mentioned for it to later become an obsession!
I have informed the campaign team and also Jeremy - they agree that there might be something in it and it has never been mentioned before because they have never been able to find a specific significance for the bible being open at that page. Fingers crossed that something comes of it!!