Olivia Newton-John's sister Rona Newton-John has died.

The 64-year-old actress' older sibling passed away on May 24 in Los Angeles surrounded by her family and friends after losing her battle with the disease.

Poignantly, the date of Rona's death would have been May 25 in Australia - where the family lived after emigrating from the UK when Olivia was six - the same date as their mother Irene.

In a post on her Facebook page, Olivia revealed: ''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.

''Rona died of a very aggressive brain tumour 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.''

Olivia - who overcame breast cancer herself 20 years ago - had cancelled all her engagements after learning of her sister's illness earlier this year, including a residency at the Flamingo Casino in Las Vegas.

The 'Grease' star - who also has an older brother Hugh - has requested that instead of flowers sent in memory of Rona for mourners and well-wishers to make donations to the Olivia Newton-John Cancer and Wellness Centre, which is starting a brain tumour wellness programme in her name.

Rona was also an actress, and had been married to the late 'Grease' star Jeff Conaway, who played Kenickie, from 1980 to 1985. They had a son, Emerson, together.