Michael C. Hall plays the nicest crime-solving serial killer you ever met on the hit show Dexter, while Jennifer Carpenter costars as his detective sister with a Daddy complex. But the on-set siblings share an even closer bond when the cameras are off: The pair are dating, sources tell PEOPLE. "I don't see what the big deal is," says a source close to the couple. "They play brother and sister on the show, but they are actors, after all." Hall and Carpenter were spotted dining at the Sunset Tower Hotel last week following the news that Dexter had nabbed a prestigious Peabody award. "It was romantic," says an observer of their back-of-the-restaurant dinner. "They were drinking champagne and there was the clinking of glasses. They were clearly celebrating." But on the set, it's back to business says a Dexter source: "Michael and Jennifer really enjoy working together, but they are always very professional on set," says the source. "But, no question, they are having fun." Reps for Hall and Carpenter had no comment.