The incident happened on January 20 around 11 a.m. inside a home on the 2300 block of Bouvier Street.
Police say officers found the girl, 9-year-old Nyssa Davis, lying in a back bedroom suffering from a gunshot wound to the right side of her head.
According to the Philadelphia District Attorney's Office, at the time of the shooting, the girl's brother and 5-year-old cousin were inside the residence with no adult supervision.
The girl was rushed to an area hospital where she later died.
Action News has learned that the 5-year-old boy fired the fatal shot. The victim's 12-year-old brother allegedly provided the boy with the weapon.
Philadelphia police said Wednesday that the 12-year-old was arrested and charged as a juvenile with Involuntary Manslaughter and REAP for his involvement in Nyssa 's death.
The district attorney's office had already filed charges against the girl's father, 39-year-old Blake Davis.
Davis is being charged with involuntary manslaughter, endangering the welfare of children, of corruption of minors, recklessly endangering another person and illegal firearms violations.
Davis' mother gathered at a vigil last week to mourn the loss of her child taken far too soon.
"These kids with these video games and these Nerf guns, just stop because my son kept thinking she had another life. And he woke up this morning saying, 'Mommy, it's going to be OK. It was an accident. She's going to come back,'" said the girl's mother.