George Clooney's Sister Adelia Zeidler Dead at 65

George Clooney's sister Adelia "Ada" Zeidler has died ... and, the star has memorialized her with a heartfelt statement.

Zeidler passed away from cancer on Friday, the actor confirmed to People ... and, her online obituary states she passed away at a hospital in Edgewood, Kentucky surrounded by loved ones.

In a statement to People, Clooney said, "My sister, Ada, was my hero. She faced down cancer with courage and humor. I’ve never met anyone so brave. Amal and I will miss her terribly."

Zeidler was born May 2, 1960 ... a little more than a year before her little bro George. She worked as an elementary school art teacher in Augusta, KY for many years.

She married retired army captain Norman Zeidler in 1987 ... and, George reportedly read scripture during the ceremony while their aunt, Rosemary Clooney, sang a romantic song. Norman passed away in 2004.

While Ada was not often in the public eye, she did attend her brother's star-studded wedding to Amal Alamuddin -- now Amal Clooney -- in 2014.

Ada's survived by her children, Nick Zeidler and Allison Zeidler Herolaga and her husband, Kenny. She was 65.