Trenton police Capt. Joseph Juniak said Wednesday that the 15-year-old first had sex with several men for money, and then gave the younger girl some cash to let a group of 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."
The child later put on her clothes and two strangers walked the crying girl home while her sister stayed behind.
The girls' parents reported them missing late Sunday afternoon. Police had just arrived at the home when the 7-year-old returned, Juniak said. She was treated at a hospital.
The teen is charged with aggravated sexual assault, promoting prostitution and other crimes. Her name was not released because of her age.
Police believe as many as a dozen people were at the party. They are trying to track down the men and boys who attended and are reviewing building surveillance videos.
Police said the girls were outside Sunday afternoon when the 15-year-old ran into two young men she knew. The young men invited her to a party at the apartment, and rather than leave the 7-year-old behind, the teen took her along.