Supermodels Gigi and Bella Hadid are opening up about discovering that they have a half-sister. The models said that their sibling, Aydan Nix, 23, was born of a brief relationship between their father, Mohamed Hadid, and a woman named Terri Hatfield Dull following Mohamed's divorce from their mother, Yolanda Hadid, in 2001. "Over 20 years ago, our dad, while single, had a brief relationship that led to a pregnancy," Gigi, 30, and Bella, 28, told the Daily Mail in a statement on Thursday, May 29.