Rhonda McCarary was to undergo surgery at Washington (D.C.) Hospital Center, where she remained in critical condition Wednesday, according to a hospital spokesperson.
Because of her medical condition, McCarary has not been told what happened to her son, her sister said.
The family had just celebrated the youngster's fourth birthday last Wednesday, Candy McCarary said.
"They went out to eat and did some fun stuff," she said.
Funeral arrangements have not been finalized. Candy McCarary said family members hope to establish a fund in the boy's name at BB&T bank in Martinsburg.
Candy McCarary said the family hopes the boy's mother will be able to recall the circumstances leading to the fire that was reported by a neighbor at 2:30 p.m. and has been ruled accidental in nature by investigators.
Rhonda McCarary was working at Heartland of Martinsburg until her shift ended at 6:30 a.m. Monday, Candy McCarary said.
The fire originated in an area next to the living room of the ranch-style home at 623 Union Ave. and was caused by combustibles that were too close to an electric space heater that was operating, fire officials have said.
Rhonda McCarary was found in the living room and had burns to about 60 percent of her body. Her son was in his bedroom and was not burned.
There were four smoke detectors found in the home, but no one has reported hearing that one was activated by the fire, investigators have said.