Wick Effect makes for a plausible explanation about what happened to Mary Reeser. According to her son, she habitually took two sleeping pills before bed.
The nightgown she wore that night was made of rayon acetate, a material flammable enough to burst into flame from contact with a cigarette.
Mary was too tired, perhaps rendered unconscious, after taking the pill as the flame grew larger. She was 170 pounds lady in her late 60s and unable to react quickly enough to save her life.