Well here goes, im unable to give my version on how i think the events took place on the night, but i do counter offer this to think over, Why were the twins spared full frontal facial fatal injuries? i think i might have a credible answer but wanted to see what the replies are.
Sheila believed her boys were possessed by the devil, but they were still her beloved boys. If Sheila killed her family, as I believe it's likely she did, she would have done so only because she was severely mentally ill. She may have been torn, on the one hand feeling that she was obeying her demonic voices and the devil, on the other hand, believing that she was performing - as Abs suggested - 'noble exorcism', an act of love to free her family from possession. But she was still the boys' mother, how could she kill them other than as she did: as quickly as possible without looking at or damaging their little faces?