Authorities lack DNA evidence, and the older girl has given varying accounts of what happened eight months ago at a party where police say she pimped out the 7-year-old and watched as she was raped by as many as seven men and boys.
Even the father of the 15-year-old has defended the three oldest male suspects, saying they tried to help the younger girl get out of the apartment.
Besides the older stepsister, two men, ages 19 and 20, and three boys ages 13, 14 and 17 also have been charged. Prosecutors have offered plea deals to all but the oldest defendant that include only one count of child endangerment in exchange for dropping the sex charges.
The older girl and the 13-year-old have accepted the deals, while decisions are pending for the others.
Early on, Mercer County prosecutor Joseph L. Bocchini Jr. said he would push to try the defendants as adults and police vowed to make more arrests. But defense attorneys questioned from the outset whether the younger girl was assaulted.
Prosecutors soon revealed in court papers that preliminary DNA tests showed no semen was found in samples taken from her.
Bocchini said that the case prosecutors were presented with initially by police changed dramatically over the course of the investigation.
"A lot stories crumbled along the way," Bocchini said, adding that the biggest stumbling block was the lack of DNA evidence. "What appeared to be the case eight months ago is not what we're dealing with now."
The teenage girl initially was charged with five counts, including aggravated sexual assault, promoting prostitution and endangering the welfare of a minor. The counts carry a sentence of up to 4 years in juvenile detention.
On Nov. 16, she was sentenced to a year in juvenile detention, where she has been since her March 30 arrest, after accepting her plea deal.
The others were each charged with aggravated sexual assault and child endangerment, including adults Gregory Leary, 20, and Timear Lewis, 19.
Leary was also charged with having sex with the 15-year-old, who was too young to consent legally.
On Friday, he was offered a plea deal that called for him to serve seven years for sexual assault for having sex with the 15-year-old and for child endangerment of the 7-year-old. If he takes the deal, he will be the only defendant convicted of a sex crime. His original charges carry up to 20 years behind bars if convicted.
He declined to comment after his court hearing. His new attorney, Shevelle McPherson, also declined to comment, saying she needed time to review it.
During a jailhouse interview, Leary previously told The Associated Press that he was at the party but "wouldn't hurt no baby."
"She's somebody's angel," he said.
The Associated Press is not releasing the names of the juveniles because they have not been made public.
Since the teenage girl's plea, the 13-year-old has also pleaded guilty to child endangerment and was sentenced to time served.
His attorney, Mark Fury, who had earlier claimed that the boy wasn't at the party, said that while the boy denied any wrongdoing, going to trial was still risky. The plea lets the boy put the case behind him and won't appear on his adult record, Fury said.
"It came out the way it was supposed to come out," Fury said.
The two other juveniles have hearings scheduled.
Lewis' attorney sought a plea offer that includes pleading guilty only to trespassing, instead of child endangerment, but prosecutors said no. So Lewis' attorney, Jason F. Hageman, is pushing for a trial.
Kevin Main, an attorney for the 17-year-old, said he is making similar request for a trespassing plea.
"Trespassing is the only thing he did wrong," Main said. "The way this case is all falling apart is certainly not surprising to me."
The incident allegedly happened in a vacant apartment at the Rowan Towers apartment complex, a public housing unit that sits in the shadow of the Statehouse, surrounded by blocks of abandoned, boarded-up homes.
Three of the defendants live there, including the 14-year-old. His mother said that early in the investigation, police began knocking on the doors of everyone in the building with teenage children.
"They didn't do a full investigation," said the mother, who spoke on condition of anonymity out of fear for her son's safety once he is released.
Police declined to respond to the mother's comments, saying they won't talk about the case until all the defendants have resolved their charges.
The mother said that at the police station, her son, who has a fourth-grade reading level, made a false confession thinking he would be able to go home if he told police what they wanted to hear.
"They tricked him," his mother said. "They just wanted to prove that they solved the case."