Police say the girl was shot by her 14-year-old cousin with an intellectual disability who got hold of the unsecured firearm inside a home on the 1600 block of 29th Street in the Strawberry Mansion section of Philadelphia.
The girl was shot in the head and died at the hospital.
On Friday, the Philadelphia District Attorney's office charged 54-year-old Twanda Harmon with Endangering the Welfare of a Child and Recklessly Endangering Another Person.
The DA's office says the gun used in the shooting was reported stolen in South Carolina.
Both grandparents, the child's mom, and another adult relative were home at the time of the shooting, along with a 9-year-old and 1-year-old.
"There is no information at this time that indicates adults in the residence besides the defendant were aware that there was a negligently stored firearm in the home," said the DA's office in a news release.
One neighbor, named Emonhny, says she saw the mother run outside with the child who had just been shot. She called 911.
"No. This is not right, this is not right. That baby is supposed to be here, playing, laughing, eating, having fun, living life," said Emonhy.
Investigators say the shooting appeared to happen in an upstairs bedroom. Neighbors say a good Samaritan pulled over and took the mom and child to Temple University Hospital.
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"This could have been prevented," said Emonhy.
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Police are urging any gun owners in the city to secure their firearms. Residents can get free gun locks from the sheriff's office.