Dead in the morning

Dr. Grant (not a medical doctor, but an Oxford don) possesses a questioning habit of mind; which gets him involved with a highly dysfunctional family helmed by a tight-fisted bullying woman. It's a situation ripe for murder, but when one finally occurs, it's not the matriarch - it's the cook.

Did the murderer make a mistake? Or could the woman have known a little too much about at least one member of the family?