Jamara Stevens was fatally shot Saturday morning with a gun that police believe was brought into the home by the mother's boyfriend a few hours earlier.
The mother was home with her four children in a master bedroom, Lt. John Walker said. Police say the shooting occurred when she left the room at about 10 a.m. to use the bathroom.
The 2-year-old pointed the loaded, cocked gun in the direction of his sister and the gun discharged, according to investigators.
The fatal bullet struck Jamara's shoulder and then traveled to her chest. Jamara was rushed to Children's Hospital of Philadelphia, but died about a half-hour later, police said.
The girl's mother was at the hospital, Walker said.
"She was distraught," Walker said.
Police believe the mother's boyfriend had brought the gun to the house and stashed it on top of a refrigerator. It's not immediately clear who brought it to the bedroom.
Homicide detectives are investigating and will forward information to the district attorney's office, police said. No charges have been filed.
The name of Jamara's mother could not immediately be confirmed Sunday evening.