There was no need for anyone to sneak in to whf in the middle of the night to enable him to kill everybody, the killer was already inside, she was having withdrawal symptoms from failing to to take prescribed drugs such as anafranil, and she was operating on a drastically reduced dosage of Haloperidol. She was possessed by devils and harboured desire to kill all people because they were bad...
It would have been relatively easy and simple for someone in Sheila's state of mind and condition, to kill her kids, kill her mother, fight with (or not) and overpower Ralph, and then kill him. ..