Isaiah Thomas Mourns Older Sister's Death, 'This Can't Be Real'

Heartbreaking news from Isaiah Thomas -- the NBA player just announced his older sister has passed away.

The 35-year-old point guard mourned the loss Wednesday evening ... posting an Instagram story tribute to LaQuisha Baldtrip.

"Ima miss you, big sis!!!" Thomas said on the post. "Damn, how you leave me too?"

"Two sisters gone!!!! This can't be real. I love and miss you, Queen."

The cause of LaQuisha's death is unknown.

This is the second time Thomas has lost a sibling -- on April 15, 2017, his sister, Chyna, was killed in a one-car accident on an interstate in Federal Way, Washington.

Thomas was on the Boston Celtics at the time ...  and still played in the team's playoff game against the Bulls the day after her death -- honoring her in the process.

He wore sneakers that read "Chyna, RIP Lil Sis" and "I love you" and led the game with 33 points.

His teammates praised Thomas for playing through his grief ... with Avery Bradley saying, "He's a true competitor, and tonight he was playing for his sister, and he was playing for his family."