Olivia Newton-John is mourning the loss of her older sister, Rona Newton-John, who lost her battle with brain cancer at age 70 earlier this week. The Grease actress made the announcement via her Facebook page on Wednesday, May 29.

"My beautiful sister Rona sadly passed on May 24 in Los Angeles. It was May 25 in Australia — which was our mother Irene's birthday," Olivia, 64, 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."

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Like her sister, Rona was also an actress. She was married to her sister's late Grease costar Jeff Conaway from 1980 to 1985.

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In lieu of flowers, the Newton-John family asks that donations be made to the Olivia Newton-John Cancer and Wellness Centre, "where a brain tumor wellness program will be started in her name," she wrote.

Olivia — who beat breast cancer in 1992 — added, "Thank you all for your kind words of love and support."

In late April, the Australian star postponed a series of Las Vegas shows in order to spend time with her ailing sister. "As a cancer 'thriver' myself, as many people are, I am very aware of the importance of love, support and family during this journey she is about to begin," Olivia explained at the time.

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