Somers firefighters found a 33-year-old woman on the second floor, where they believe the fire started.
Fire officials said the fire was under control by 9 a.m. The cause remains under investigation.
Karen Harvey of Enfield identified the victims as her granddaughter, Kerra Colgan, and her daughter, Katie Colgan.
Both victims were taken to Baystate Medical Center in Springfield. Harvey said Kerra Colgan was transferred to Shriners Hospitals for Children in Boston and Katie Colgan was transferred to Massachusetts General Hospital, also in Boston.
Shriners Hospitals for Children in Boston would not release Kerra Colgan's condition. Massachusetts General Hospital said that Katie Colgan was in fair condition.
The passerby asked to remain anonymous, a relative of his said.
"He spoke to the family this morning, and won't talk to anyone in the media," the relative said. "He wants privacy on this."
The family had six pet ferrets. Two died, officials said. Kerra Colgan's kitten, Cupcake, survived.
School bus driver Lori-Anne Davis said she normally picks up Kerra Colgan around 8:30 a.m., but couldn't get through Wednesday morning.
"She is very, very sweet, a cute little blond girl that loves to run around and play. She's into everything kids are into. She loves those little rubber band bracelets, playing the guitar and, of course, her kitty, Cupcake, which I have in the container here," Davis said.
Written by Hartford Courant