ABC News said Olivia Newton-John, 64, postponed a stage show in Las Vegas last month after her sister was diagnosed with a brain tumor.
The model and actress, who was married to Newton-John's "Grease" co-star Jeff Conaway from 1980-85, died May 24. Conaway died in 2011.
"My beautiful sister Rona sadly passed on May 24th in Los Angeles. It was May 25th in Australia -- which was our mother Irene's birthday," Olivia Newton-John, a breast cancer survivor, said in a Facebook posting Thursday.
"Rona died of a very aggressive brain tumor and mercifully suffered no pain. She was surrounded by the love of her four children -- Fiona, Brett, Tottie and Emerson, and her wonderful friends. I will miss her forever -- my beautiful, smart, talented, funny, brave sister Rona."
Rona Newton-John's memoir, "The Dark Side of Charisma," is to be published this summer, ABC News said.