The Monday fire was set by her 8-year-old grandson who was playing with a cigarette lighter, The Miami Herald reported Tuesday.
Manuela Morales, 88, died as flames and smoke swept through her fifth-floor apartment home in south Florida, keeping her family at bay.
"They tired to rescue her, but were overwhelmed by the smoke and had to get out," said Detective Robert Williams, a police spokesman.
Williams said the boy was playing with a candle and the lighter when he started the fire. His older brother rushed into the room and tried to rescue Morales, but he couldn't free her from the ropes tying her to the chair.
Family members said the victim was immobilized with rope to keep her from injuring herself.