Their parents, none the wiser, thought maybe they had run away.
"We're talking about a kid who told her sister to go into an apartment and let people rape her," said Trenton police Capt. Joseph Juniak. "It's unfathomable."
The teen has been charged with aggravated sexual assault, promoting prostitution and other crimes. Her name was not released because of her age, but the county prosecutor plans to ask the court to try her as an adult. In the meantime, she is being held at the Mercer County Youth Detention Center.
The 7-year-old had wanted to tag along because she was worried about the 15-year-old's safety, Mayor Doug Palmer said.
When the girls didn't return home by 4:30 Sunday afternoon, their parents called police, believing the older one had run away from home and taken her younger sister with her.
In fact, they were down the street inside a 13th-floor apartment at Rowan Towers, a high-rise complex so dangerous that police are hired as security guards at night.
"They keep it clean on the outside, but it's what's on the inside that you have to worry about," said neighbor William Johnson, who says police are coming out of the building all the time.
Inside apartment 13-C, police said, the 7-year-old was soon left alone as her sister headed to a back bedroom to sell sex to several men. When she came out into the living room, she handed her 7-year-old sister money and encouraged her to let the men touch her.
"It went from touching to straight out assault and rape," Juniak said. "They threatened to kill her if she screamed or told anyone."
Afterward, the child put on her clothes and left. Her sister stayed behind with the men.
Two women found the child crying outside the apartment and walked her home, where police were waiting.
The child told them what happened and was treated at a hospital.
When police located the 15-year-old later that night, she also told them what happened and was arrested.
Palmer said the crimes are among the worst he's seen in 20 years as mayor.
"It's sickening," he said. "The police are taking this personal. I know there's a place in hell for all the people that participated in this and I'm sure they will get there."
"Personally, as a father with a 7-year-old daughter, I can't imagine the horror," Palmer added.
Lauren Kidd, a spokeswoman for New Jersey's Children and Families department, said state and federal confidentiality laws prohibit the agency from commenting about possible prior involvement with the family. But Juniak indicated the department may have had previous contact with the older girl.
Police are now scouring video surveillance from lobby and elevator cameras to try to identify everyone at the party. They believe there were about a dozen people in the apartment, mostly teenage boys and men who police say likely broke in - a fairly common occurrence in the crime-plagued neighborhood that sits in the shadow of the Statehouse's golden dome.
Last week, police responded to a home invasion there and a shooting just outside the lobby.
Police Director Irving Bradley Jr. said the building's management company, Interstate Realty Management Co., has been working with police to curb the violence.
"This is incredibly disturbing," said Laura Zaner, a spokeswoman for IRM.
Two private security guards man the lobby doors during the day. At 5 p.m., two police officers take over.
Bradley said the company is installing more cameras and had just hired a third officer to work the night shift to allow two officers to do hourly hallway patrols. He said Sunday may have been the first day they were supposed to have started the patrols.
Chalia Johnkins, who lives around the corner from the Towers, said gatherings of men are commonplace and police should have known something unsavory was happening.
"The police who were supposed to be on patrol should be held responsible," she said. "They could have prevented this. These weren't regular guards. They were police and they still didn't see the baby crying?"
Annette Lartique, the city councilwoman who represents the area where the crime occurred, said the community would expect nothing less than the prosecution of everyone involved to the fullest extent of the law.
"I know we are going to send a message on this one," she said. "Everybody will pay a price - from the person who opened the door to the person who pushed the elevator button."