When JS pulled out her gun she said her son, Beau, threatened to hit her. When JS killed her son she said Beau put up a pretty good fight.
IMO this shows JS was cognizant of reality and knew what she was doing. IMO she crept up behind her daughter, Calyx, to stop a recurrence of the resistance she got while killing her son. Isn't being cognizant a sign of sanity?
Beau was also killed outside of the house so Calyx didn't here the gunshot---she was worried one of them would hear it and "thwart" the plan...