Bagley, Coretta Scott King's elder sister, died Saturday.
The Atlanta Journal-Constitution report of her death didn't give an age, but other media sources said she was 86.
Bagley was an active board member of the King Center in Atlanta and spent 26 years as a professor at Cheyney University. In a family statement, nephew Martin Luther King III said his aunt died peacefully, although he didn't say what caused her death.
"I and my family are deeply saddened by the death of our beloved Aunt Edythe Scott Bagley," King said in the statement. "Our Aunt Edythe was a vibrant, brilliant woman, and always a source of strength and wisdom four our mother during the difficult challenges of the civil rights movement. We will miss her dearly, and she leaves a great void in the hearts of our family and her many friends and colleagues"
Bagley recently completed a biography of her sister, Coretta. The University of Alabama Press will publish the book next year.
Bagley was born in Marion, Ala., and was the oldest of three surviving children born to Bernice McMurry Scott and Obie Scott.
She was a high school valedictorian in Marion and she was the first black student to attend Antioch College in modern times.
Survivors include her son, a brother and a nephew.