Shaun's message all about thanking the fans for their support during this trying time.
"Thank you for all your love and good wishes for David," Shaun wrote. "It means the world to all of us."
Thank you for all your love and good wishes for David. It means the world to all of us.— Shaun Cassidy (@shaunpcassidy) November 18, 2017
Shaun's message was sent Saturday evening, and Cassidy's condition has reportedly worsened since.
The two brothers were both in the entertainment industry, with Shaun starring on The Hardy Boys/Nancy Drew Mysteries and Cassidy starring on The Partridge Family. They also both had successful music careers.
The two brothers had collaborated musically numerous times and even starred in the Broadway play Blood Brothers together.