Does Unauthorized Biographer Know Oprah's Big Secret?

Jan. 24, 2011— -- As Oprah Winfrey approaches her 57th birthday on Saturday, it seems inconceivable that there is something her audience doesn't know about her.

For 25 years, Oprah has spoken candidly about her weight, the sexual abuse she endured, a teenage pregnancy, her favorite things - seemingly everything from her favorite book to telling Barbara Walters flat out, "I am not a lesbian."

So what could the big secret be?

FOR AN UPDATE ON WHAT THE SECRET IS CLICK HERE

"I was given some news that literally shook me to my core," Winfrey says in a promotional advertisement for today's show. "This time, I'm the one being reunited. I was keeping a family secret for months, and on Monday you're going to hear it straight from me."

Clues to the Big Reveal

So we know it's a reunion and the footage shows someone from behind walking out onto the stage.

Who could it be?

CLICK HERE to see photos of Winfrey through the years.

Last year "Oprah," Kitty Kelley's unauthorized biography hit Amazon's bestseller list.

The media mogul didn't cooperate with Kelley on the book and Winfrey's best friend, Gayle King, wryly told "Good Morning America" that it was unlikely to "make her book club selection."

Kelley culled close to 3,000 transcripts of Winfrey's past interviews and conducted 850 of her own.

The biggest bombshell from the book was the claim that Kelley knows the identity of Winfrey's biological father.

The author says that on July 30, 2007, one of Winfrey's relatives told her the name and details surrounding her biological father.

Kelley said she wouldn't reveal the name until Winfrey's mother, Venita Lee tells her daughter the news herself, something she has refused to do for decades.

But Oprah's secret may not even involve her biological father but an unknown half-sister born to her mother.

Kelley told Time Out Chicago blogger Robert Feder that Winfrey would be reunited with a woman named Bunny, her half-sister, on the show today.

The woman was later given up for adoption.