The incident happened on the second floor of the Edenwald Houses at 1131 East 226th Drive around 9 p.m.
Witnesses say the girl landed on a first-floor air conditioner, then bounced to the ground.
She was taken to Jacobi Medical Center, where she is listed in stable condition with a fractured skull.
Police now say that after re-interviewing the family, it was the girl's 9-year-old sibling who dropped his little sister out the window.
The boy was holding the baby at the window, trying to get his mother's attention. She was outside on the street at the time.
That's when the boy accidentally dropped the child out the window.
Two uncles and an adult woman were inside at the time. No charges have been filed.
All of the apartments of the three-story building appear to have window guards, but the girl somehow squeezed through the bars.
The Edenwald Houses is the largest NYCHA development in the Bronx with 40 buildings and 2,036 apartments.
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