Officers found the child and the 38-year-old woman unresponsive in the home in the borough's Wakefield section just before 8:30 p.m. They were pronounced dead at the scene.
It is unclear how long the girl was left with the bodies of her mother and brother. She was taken to Montefiore Medical Center for further evaluation.
While the investigation continues, the deaths are being treated as non-suspicious at this time, authorities said.
The woman used a wheelchair, and the boy had a severe form of autism and had disabilities.
The Medical Examiner will determine the causes of death.
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