Chyna Thomas died in a one-car accident in Federal Way, Washington's Interstate 5 on Saturday morning.
The Celtics held a moment of silence before the game in honor of Thomas's sister, and the point guard received a loud cheer of support from the home Boston fans when he was introduced. Thomas went on to lead all scorers with 33 points but the Bulls took Game 1 106102.
Head coach Brad Stevens said earlier in the day that Thomas intended to play in the game despite the trying circumstances, according to Chris Mannix of The Vertical.
"Isaiah Thomas is struggling...tragic circumstances he is going through...his intention is to play," Stevens reportedly said.