Olivia Newtown-John's older sister Rona has died after a short battle with brain cancer. She was 70.
Rona, British-born and Melbourne-raised model and actress, was diagnosed with cancer in April.
Olivia, 64, who is in remission after her 1992 breast cancer diagnosis, announced her sister's illness in a statement last month.
"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 said.
"I want to thank everyone in advance for respecting our privacy during this difficult time."
Rona's condition deteriorated so fast Olivia was forced to cancel her upcoming concert dates in Las Vegas.
Rona married Olivia's Grease co-star Jeff Conaway in 1980 and they had a son, racing car driver Emerson Newtown-John. The marriage ended in divorce in 1985.
Rona later married Melbourne restaurateur Brian Goldsmith and had daughter Tottie Goldsmith.