Fans were quick to rush to Twitter to send their thoughts and prayers.
"I’m so very sorry about the loss of your brother. Love and peace to you," one fan penned.
Another remarked, "Much love to you and your family. Thank you for sharing this lovely tribute with all of us."
Hugh was a doctor and his cause of death is yet to be revealed.
This is not the first time Olivia's family has been rocked with death.
In 2013, her sister, Rona, passed away from cancer.
Cancer has rocked ONJ twice now, inspiring her to open her foundation called Olivia Newton-John Cancer Wellness & Research Centre.
Speaking about how she deals with such adversity, ONJ has said: “Staying positive and seeing good things ahead of you is really important to me and for anyone who's going through cancer or any difficult illness.”
WATCH: Olivia Newton-John speaks candidly about her breast cancer battle