Olivia Newton-John, 64, is a breast cancer survivor.
"I received the very sad news that my beautiful sister, Rona Newton-John, has been diagnosed with brain cancer. In light of this news, I have decided to postpone my forthcoming Las Vegas residency to spend time with her and our family," Newton-John said in a statement to the Las Vegas Sun.
"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. I want to thank everyone in advance for respecting our privacy during this difficult time."