Olympic gold medalist Dominique Moceanu discovered at 26 that she had another sister, given up for adoption at birth.

Moceanu reveals the family secret in her new memoir, "Off Balance," being released Tuesday. She tells ABC's "20/20" in an interview scheduled to air Friday night that she learned of her sister in December 2007, when the young woman sent her a letter and a package of photos. Moceanu calls it "the biggest bombshell of my life." The young woman, six years younger than Moceanu, has no legs, and Moceanu says doctors recommended she be given up for adoption because her care would be expensive and her parents had little money.

Moceanu was part of the squad that won gold in Atlanta, the U.S. women's only Olympic team title.