Christine Matsil, 82, and her son, David 54, were found dead in his bedroom at their home in North Bellmore.
The blaze started at 11:15 a.m., when Matsil's husband, Max, also 82, kindled a fire in the den fireplace on the ground floor, police said.
After he went upstairs, he and his wife heard the smoke alarm. Embers from the fireplace had landed on nearby papers, igniting the blaze.
The couple went down to the den, but were repulsed by the flames.
Max then tried to go upstairs to rescue his son, but was unable to get to the second floor - and lost sight of his wife.
Police were the first to enter the house. They found Max lying on the ground floor and carried him out.
In the bedroom, they found David dead in a hospital bed and Christine's body on the floor.
Max was taken to Nassau County University Medical Center in stable condition with smoke inhalation and burns.
Police described the younger man as mentally handicapped. Neighbors said he had had Down syndrome.
"He's been bed-ridden for about three or four years," said John Bongiorno. "They were just taking care of their kid. The mother was a retired nurse so she knew how to do it. They were just dedicated."
Christine had recently fallen and injured her hip, neighbors said.
One, Robyn Martin, visited Max in the hospital's intensive-care unit last night.
"He's doing rather well, actually. He's really coherent. He knew me right away," she said. It took 75 firefighters to battle the blaze. Three suffered minor injuries.
