The singer's 36-year-old sibling had been battling brain cancer for over a decade, and his illness prompted Cook to postpone some concerts on his U.S. tour in March, when he took a turn for the worse. Adam's health improved and Cook returned to the road in April.
But on Saturday, friends and family gathered by his bedside at a hospice in Terre Haute, Indiana, as Adam's ailment became more severe. He passed away at 1 a.m. on Sunday.
A family friend tells RadarOnline.com, "Adam passed away peacefully. He was surrounded by his family, including his wife Kendra, his children, his father and his step-mom, among others."
Cook was due to fly home on Saturday to be with Adam in his final hours, but was convinced by his family to honor a prior commitment to attend the 12th annual Race for Hope in Washington, D.C., on Sunday morning.
The insider adds, "The family thought that Adam would go much sooner than this, actually. That's why David left his tour a few weeks back so that he could be with Adam. They said their goodbyes then."
Cook pressed ahead with the Race for Hope, in which he also participated, and, during a heartfelt speech after the charity run, he fought back tears as he broke the news of his family tragedy.
He also paid tribute to the brain cancer sufferers who took part in the event and vowed to continue supporting the cause year after year.
He told the crowd, "I'm affected by this disease. I actually lost my brother yesterday (sic) to a brain tumor and I couldn't imagine being anywhere else right now.
"I go on stage every night and make noise -- that's my job and it's an amazing job... and I am consistently amazed at what survivors like what the people in this audience do and go through... I lost one today, but I've gained 9,000 new friends and I will be here every year that they will have me."
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