"It was May 25th in Australia - which was our mother Irene's birthday," the Grease star wrote. "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."
Newton-John canceled a scheduled appearance in Las Vegas last month to be with her sister after she was diagnosed with the brain tumor.
In honor of her sister, Newton-John (who survived cancer herself 20 years ago) is starting a brain tumor wellness program in Rona's name at the Olivia Newton-John Cancer & Wellness Centre.