"It was the biggest bombshell of my life," the Olympian tells 20/20 in an interview set to air Friday night. "I had this sister that was born who was given up for adoption, and I never knew it."
Estranged from her parents Dmitry and Camelia after becoming legally emancipated in 1998, the family later reconciled and Dmitry walked Moceanu down the aisle at her 2006 wedding to Michael Canales, a doctor and fellow gymnast.
But it was when her father was sick with cancer in 2008 that a then-pregnant Moceanu got the surprise news.
She received a package with a letter and photos of a young woman who resembled her younger sister, Christina, she tells 20/20.
According to the 20/20 interview, the gymnast confronted her parents – who then told her the baby was born the day after her sixth birthday. The child would require expensive medical care and, since the family was poor and couldn't afford it, they gave her up for adoption.
Another ShockerMoceanu, who recounts the story in her upcoming memoir Off Balance, has since reached out to her sister, who confirmed their relation – and also clarified her health situation: she had no legs.
According to Moceanu, she explained, "'People forget that within minutes of meeting me.' "
The sisters have met and subsequently developed a real bond and have enjoyed discovering their similarities. "The tones in our voices, our handwriting, the way we laugh and chuckle. It's mind-blowing," Moceanu says.
Moceanu and her husband now live in Cleveland with their daughter Carmen, 4, and son Vincent, 3.