The girl was shot once in the face and was pronounced dead at the scene by medics.
The semi-automatic pistol that was used in the shooting was discovered to be that of their mother's boyfriend who fled the scene after the shooting.
He was later apprehended late Saturday evening is being interrogated by the Police.
Sgt. Eric Gripp said police were trying to determine who was in the home when the shooting occurred, CBS Philadelphia station KYW reported.
Several neighbors identified the girl’s mother as Tera Riddick. They said she and Phillips are the parents of six children, four daughters and two sons.
Riddick was known for being a protective mother who never let her children play outside the home, which is in Philadelphia's violent Kensington neighborhood.
Police said the weapon was recovered and no arrest has been made yet.