According to the News Tribune's Craig Sailor, Chyna J. Thomas, 22, was pronounced dead at the scene of the accident after the 1998 Toyota Camry careened into a barrier after the car drifted into the shoulder.
"That vaulted the vehicle head-on into the sign post," State Patrol spokesman Trooper Nick King said, per Sailor.
According to Washington State Patrol, Thomas was not wearing a seat belt at the time of the accident.
"We are terribly saddened by the tragic loss of Chyna Thomas," the Celtics said in a statement Saturday, according to ESPN.com's Chris Forsberg. "The thoughts and prayers of the entire Celtics organization are with Isaiah and his family."
NBA commissioner Adam Silver also released a statement expressing his condolences:
NBA Commissioner Adam Silver released the following statement regarding the passing of Chyna Thomas, sister of Isaiah Thomas of the Celtics. https://t.co/iPOFte2jzM2017-4-16 00:48:05
Thomas and the Celtics are scheduled to open their first-round playoff series against the Chicago Bulls on Sunday at 6:30 p.m. ET at TD Garden.
Adam Himmelsbach of the Boston Globe reported members of Thomas' inner circle will head to Boston on Sunday morning to be with the point guard.