Chyna Thomas, 22, died at the scene of the accident on Interstate 5 in Federal Way, Washington, police said.
A source told ESPN’s Jeff Goodman that Isaiah Thomas received news of his sister’s accident after taking part in Celtics practice in the afternoon.
According to the source, Thomas is expected to play when the Celtics open their first-round series against the Chicago Bulls on Sunday in Boston.
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.
The news of Chyna Thomas’ death was first reported by KIRO 7 in Washington.