"As you may know, my brother Tom underwent quadruple bi-pass (sic) surgery. He's on life support right now & fighting to hang on," Donnie, 60, wrote in an emotional tweet.
"We don't know what the future holds, but it's in the Lord's hands. Our entire family would appreciate your thoughts & prayers on behalf of my sweet brother."
Tom, who at 70 is the second eldest Osmond sibling, mostly stayed out of the family's large spotlight, and instead spent 28 years working for the U.S. Postal Service, before retiring in 2014.
He did make a few public appearances, most notably showing up in the Christmas specials from the 70s and early 80s.
Tom, along with his older brother Virl, was born with a severe hearing impairment. Their younger brothers originally formed the musical group in part to support Tom and Virl in buying hearing aids.
Sharing her thoughts on Facebook, sister Marie called her deaf brothers her "heroes."
"Can you imagine being born into a family of singers?" she asked. "I was always amazed by their strength."
In a lengthy post, 58-year-old Marie, who is no stranger to family tragedy, revealed doctors are not sure how Tom will do in his recovery, and her tone seemed grim and reflective.
"My greatest wish for him is to enjoy a second chance at life and have many more years on this earth with his family, but the Lord's plan will always be done," she wrote. "I am comforted to know that no matter what happens, our bond is eternal and families are forever!"
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