Chyna Thomas died at the scene of the accident on Interstate 5 in Federal Way, Washington, police said. She was 22.
A source told ESPN's Jeff Goodman that Isaiah Thomas received news of his sister's crash after he participated in Celtics practice in the afternoon.
Thomas was talking to a reporter after practice when Avery Bradley, a fellow Tacoma native, emerged from the trainer's room and pulled him away. Thomas never returned to engage in other scheduled media requests.
A source close to Thomas told ESPN the plan as of Saturday night is for Thomas to play Sunday evening when the team opens its first-round series against the Chicago Bulls in Boston.
Isaiah Thomas' father, James, told the Boston Herald that his son hadn't decided when he would fly home to Tacoma, Washington, to be with family members.
"It's a crucial time for our family right now," James Thomas told the Herald.
In a statement released Saturday, the Celtics extended their condolences to the Thomas family.
"We are terribly saddened by the tragic loss of Chyna Thomas," the Celtics said in the statement. "The thoughts and prayers of the entire Celtics organization are with Isaiah and his family."
NBA commissioner Adam Silver also released a statement on Chyna Thomas' death.
"The NBA family mourns the tragic passing of Chyna Thomas, and we send our deepest condolences to Isaiah, his family and the Celtics organization during this difficult time," Silver said in the statement.
Police said the accident occurred at around 5 a.m., when Chyna Thomas' car suddenly drifted onto the left shoulder of the highway and traveled about 50 feet before hitting a large metal pole.
Police said Chyna Thomas was not wearing her seat belt.
Isaiah Thomas, who was named an NBA All-Star for the second consecutive year, averaged 28.9 points and 5.8 assists in leading the Celtics to the top seed in the Eastern Conference.
Information from ESPN's Jeff Goodman and Chris Forsberg was used in this report.