Grease star Olivia Newton-John has called off her upcoming Las Vegas residency to spend time with her sister, who is battling brain cancer. The singer/actress was due to begin a series of shows at Sin City's Flamingo hotel this summer (13) but she is postponing the dates to support her elder sibling Rona, an actress who was once married to Grease actor Jeff Conaway. In a statement, Newton-John says, "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. "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." The 64-year-old star was diagnosed with breast cancer in the early 1990s, but she beat the disease after undergoing chemotherapy treatment and a mastectomy.