Authorities say the teen brought the young girl to a party on the 13th floor of the Rowan Towers along the 600 block of West State Street on Sunday afternoon.
Over the course of the afternoon, the teen was allegedly paid for sex by several men and teens before allowing her stepsister to be gang raped, police said Wednesday.
As many as seven men and boys took turns having sex with the adolescent as the teen watched, authorities said. She was then allegedly paid for the rape.
Trenton Police Capt. Joseph Juniak says the men held the girl down on the bed and "threatened to kill her" if she put up a fight.
The girl left the party crying and was found by two women outside the apartment building. The girl's stepsister had stayed behind.
The women walked the girl home where they were met by police officers, authorities say. Police had been searching for the girls after their parents reported them missing.
Police say they were able to retrieve DNA evidence after administering a rape kit on the girl at the hospital. But since the attack, no arrests have been made.
The disturbing case has city officials disgusted. Trenton Mayor Doug Palmer pleaded with residents to come forward with names for the men he calls "pedophile, pervert animals."
Building security allowed NBC Philadelphia to visit the apartment where the alleged party and rape happened, but there was no answer at the door.
Trenton authorities say in addition to the security, the building is patrolled by two uniformed officers between the hours of 5 p.m. and 3 a.m. because of crime in the area. However, those officers only command posts on the first floor.
The rape was believed to have happened between the hours of 4:30 and 5 p.m., so authorities most likely wouldn't have been present.
Investigators are looking into elevator security tapes to see if they can yield any clues.
In the meantime, the teen has been charged with Aggravated Sexual Assault, Promoting Prostitution and other crimes. Her name was not released because of her age.