Pickett was a student at Wheaton-Warrenville South High School. She and Leath are both from Warrenville, Ill.
"It's really hard right now. Not only for our family, but for my sister as well," Victor Dulceak, Pickett's uncle, said.
Prosecutors say Leath was on a drinking binge-- doing shots on Thursday afternoon and evening -- before crashing his car around 10:30 p.m. on Shaffner Road near Mack Road in Winfield. The car struck several trees and rolled.
Police say Leath's blood alcohol level was more than two times the legal limit. Pickett's family says they were told she was drinking, too.
"We want to find out where she was last night, drinking underage at a bar in West Chicago," Dulceak said.
Pickett suffered head injuries and was pronounced dead at Central DuPage Hospital.
Friends say the teenager recently began working at Happy Paws Pet Resort, a pet boarding facility.
"She seemed to like it very much. She really liked working with the animals," John Marak, co-owner of Happy Paws, said.
Pickett's school released a statement, "[She] will be missed by students and staff alike. We extend our deepest condolences to Stephanie's family."
The school had a crisis team in place for students needing to grieve.
Both directions of Shaffner Rd. were closed for about five hours while sheriff's investigators reconstructed the crash.
Officials said Leath has been arrested 12 times since 2004.
His bond was set at $1 million, and he will next appear in court on May 20.