It was Winfrey's assistant that spilled the beans to the talk show host last fall, but her half-sister, Patricia, had apparently known since 2007.
Born in Milwaukee, Wisconsin in 1963, Patricia was put up for adoption at birth. All her life, she tried to find her birth mother.
"I had some disbelief that she didn't mean to put me up for adoption," Patricia said. "I had just always wished that my birth mother was going to come and get me."
However, Patricia's birth mother did not want to be revealed. So it was through what Winfrey calls a "God moment" that Patricia learned the truth.
"We realized that Oprah could be my sister," Patricia said.
Last Thanksgiving, she finally met her famous sister.
Patricia, along with her two children, appeared on Winfrey's show Monday. The talk show host said one of the most impressive things about this discovery was that Patricia avoided the media with the news.
"She never once thought to go to the press," Winfrey said. "She never thought once to sell this story."
Winfrey said she was 9 years old and living with her father when her mother, Vernita Lee, had the daughter and gave her up for adoption. She says she never even knew her mother was pregnant.
Winfrey, who said DNA tests confirmed that the two are half-sisters, met with Patricia and their mother in a pre-recorded segment of the show.
Winfrey said documents from the girl's birth reveal that Lee gave up the baby for adoption because she did not think she could get off welfare if she kept the child.
Patricia is 9 years younger than Winfrey.
The Associated Press contributed to this report.