'The Masked Singer': Donny Osmond Is Still Mad at Sister Marie for Almost Ruining the Surprise! (Exclusive)

The famous crooner was underneath the Peacock mask all season -- but his family almost blew it!

Donny Osmond was the runner-up on season one of The Masked Singer -- but his family almost blew the big reveal!

The legendary crooner was unmasked as The Peacock on Wednesday's season finale -- coming in second to rapper T-Pain, who was behind the Monster mask -- but when Osmond sat down with ET's Kevin Frazier, he revealed that he's still a little upset with sister Marie for almost ruining the surprise, when she hinted during a recent interview that he might be under the mask.

"Don't even bring Marie's name up!" he said with a laugh. "I was so mad. [That's] the last time I'm going to trust Marie with a secret. To her defense, she was trying to be funny... [but] I called her up: 'How could you do this to me?'"

In fact, when Osmond was asked which mask his younger sister might don if she ends up on a future season of Masked Singer, he didn't hesitate: "Monster!" he exclaimed. "Typecasting!"

However, the sibling spat hasn't kept the pair from performing their headlining show in Las Vegas almost every night. In fact, Osmond was flying back and forth every day while filming The Masked Singer just to keep the secret. But are the rumors true that the pair are thinking of shutting down their Sin City act?

"Those are rumors right now," he explained. "Marie and I have to really sit down and think this through."

The life-long performer found a new audience on the singing competition, noting that the show was an opportunity to "reinvent" himself.

"Think about it: I've been reinventing myself for 56 years," he said. "That brings longevity... I'm like a bad rash, I don't go away!"

And while his sister was in on the secret, there was another family member who also predicted the big reveal: his 6-year-old grandson Truman!

"He calls me and says, 'Grandpa, I love The Peacock, but he sounds so much like you!" Osmond recalled. "I felt so bad, because I couldn't tell him. He says, 'Grandpa, I think it's you,' I said, 'We'll find out! I don't think so.' I don't think he's going to trust me the rest of his life!"

The Masked Singer will return for season 2 on Fox -- see more in the video below!