The case is unusual because prosecutors typically need a blood or breath test immediately after an accident and a cop's firsthand observations of a driver to prove that someone was driving under the influence.
Lugo wasn't identified as a suspect until months after the crash, so there were no cops to see her demeanor and no chemical tests to show if she had alcohol in her system.
"It's very difficult for them to prove a DUI a year later," said Steven Kellis, a Philadelphia defense lawyer who specializes in DUI. "It's a tough case."
A police supervisor declined to discuss Lugo's case, but her arrest affidavit provides details from a 15-year-old witness who said she was with Lugo in the car the night of the crash.
According to the affidavit, the witness told police she was at a party with Lugo for three hours that night, during which time Lugo drank 7 1/2 beers, more than enough to put someone over the legal limit. The teen said she then got in the car with Lugo, who was allegedly driving "real fast."
After hitting Acevedo, the girl said, Lugo threw beer bottles out of her car window as she drove away, according to court documents.
Lugo's attorney, Guy Sciolla, said the case is "completely based on anecdotal evidence."
"Here, the government's relying on a juvenile who may or may not know what the signs of intoxication are," Sciolla said. "In this case, it's just going to be her word against Sylvia's. I don't think it's the strongest situation the government would like to present." Lugo remains in jail on $500,000 bail.
Betzy Cruz, Acevedo's cousin, said the family was upset when they learned that if prosecutors can't prove that Lugo was driving impaired that night, she would likely face only a year in prison if convicted.
"We are happy because finally, someone is going to pay and be held accountable for her death," Cruz said. "At the same time, it brought it all back and started a new chapter of pain and suffering because we have to deal with the fact that she might not do any time or that she might only do one year."