The patrol says 17-year-old Janesah Goheen died Monday at Harborview Medical Center in Seattle, where she was taken for treatment after a two-car collision on State Highway 20.
Her sister, Janeah Goheen, died at the scene. An 18-year-old friend, Alysha Pickler, was also injured in the crash.
The driver of the other vehicle, Ira Blackstock, is being held in Skagit County Jail.
Samantha Blankenship passed the accident that Halloween night without knowing at the time that her friends were involved.
"Passing through it, going back through it, was even more heartbreaking," she said.
Now Blankenship and others are struggling to deal with such and enormous loss.
"You just cannot even wrap your head around the fact they're not there anymore, and it hurts to think about because bad things shouldn't happen to such good people," Blankenship said.
Trooper Mark Francis said Blackstock showed signs of impairment. The Skagit Valley Herald says Blackstock has already been charged with one count of vehicular homicide and two counts of vehicular assault.