What Is Oprah's Family Secret?

Oprah Winfrey - TCA: Own Network -  2011
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The talk show host will reveal news Monday that she says "shook me to my core."

What is Oprah Winfrey's big secret?

The talk show host has promised to reveal a whopper Monday on her show, in its 25th and final season.

Biographer Kitty Kelley says that Winfrey will reveal she has a half sister named Bunny, whom her mother, Vernita Lee, put up for adoption because she couldn't afford to raise her.

Her father, Vernon Winfrey has no idea. "I don't have the least idea of what she has in mind," he told the Associated Press Sunday from Tennessee.

Celeb blogger Perez Hilton is among the camp that believes Winfrey will announce Vernon isn't her biological father. (After she was born to unwed teenagers, she was raised by various family members, including a grandmother, her mother, her father and stepmother, across several states: Mississippi, Wisconsin and Tennessee.)

Harpo Productions declined to provide details to the AP.

In a promotional spot that shows a shadowy person joining her on stage, Winfrey calls the revelation a "miracle.

"I I was given some news that literally shook me to my core. This time, I'm the one being reunited," she said in a teaser clip (watch below). "I was keeping a family secret for months, and on Monday you're going to hear it straight from me."

Winfrey has shared private information on her show in the past, including admitting she was molested as a child, and that she considered suicide after discovering she was pregnant at 14.