Chyna Thomas died at the scene of the accident after her car wrecked around 5 a.m.
A witness said the car was traveling normally at first before it started drifting onto the left shoulder. The car went about 50 feet off the roadway before colliding with a Jersey barrier.
Chyna Thomas was reportedly not wearing her seatbelt.
The Celtics will play the Chicago Bulls on Sunday in Boston. Thomas is still in Boston and did not find out about his sister's death until after practice, according to Mark Murphy of the Boston Herald.