From the IMDB:
'Lost' Siblings Start Dating
Actors Maggie Grace and Ian Somerhalder, who play spoiled step-siblings Boone and Shannon on hit TV series Lost, are dating off-screen. The celebrity couple are regularly spotted out together in Los Angeles, but have tried to keep their budding relationship under wraps. A source confirmed the romance to American publication Us Weekly adding, "They stay out of the limelight, but they spend a lot of time hanging out and working out together at Gold's Gym (in Venice Beach, California). They kiss and hold hands when they come and go from the gym. You can tell they're a couple." The pair were romantically involved in a flashback sequence on Lost in its first season.