I think too much has been made of the 'ritualistic cleansing' theory offered by some 'experts' who have appeared on documentaries or used as sources in books on this case, surely if Sheila's mind was working that way after the shootings then she would have chosen a symbolic place to actually commit suicide-for example, the twins room? The choice of a nightdress is, in my opinion, an odd thing to choose to wear if in this symbolic mindset, unless she chose to shoot herself whilst lying on a bed. The idea that she soaked her knickers for any other reason than to rid them of menstrual blood is, in my view, clutching at straws!
Sheila either committed the murders and escaped being covered in gun residue/victims blood etc. or she was murdered with the rest of the family IMO.