According to two inside sources, Mia Maestro, a drop-dead-gorgeous Argentinian actress (and the spitting image of Lena Olin –it’s eerie) has been cast as Nadia, Sydney’s sister. From what I understand, she’s signed on for three episodes this season and will be coming back next year. Now, it turns out (Here comes the storyline so don’t read if you don’t want to know!) that Sloane and Irina did have an affair, and did have a child together, but that child is not our beloved Sydney. … Sloane and Irina’s love child is Nadia, and only Sloane knows where she is. And Sloane will, of course, use all this to his advantage. From what I understand, Mia probably will be a regular next season, and Melissa George will step down to recurring.
Source: Kristin on E!Online