(CNN) -- Dr. Martin Luther King Jr.'s sister-in-law, Edythe Scott Bagley, has died of natural causes at her Pennsylvania home, her family said Monday.
Bagley, a retired college theater professor, was Coretta Scott King's older sister.
"Our Aunt Edythe was a vibrant, brilliant woman and always a source of strength and wisdom for our mother during the difficult challenges of the civil rights movement," Martin Luther King III said in a written statement. She died Saturday, he said.
Coretta Scott King, the matriarch of the King family, was 78 when she died five years ago.
Bagley, whose age was not given in the family statement, served on the King Center's board of directors. She also often represented the King family in public appearances.
She recently completed a biography of her sister that is set to be published by the University of Alabama Press next year, the family said.
Her funeral is scheduled for Friday in Westchester, Pennsylvania.