I liked it on Buffy that time, during a costume party at the Bronze, it might have been halloween, there was a Luke and Leia dancing in the trhong, way to close and way too dirt... Which iswhen Willow says "Good lord! Do they know that they're brother and sister?"
Meanwhile off in Ugly Betty, when the Secretary thought that she was the Prince Charming Publisher's halfsister, and he says "Oh my, You're my sister? When I think of all the sex we've had over the years." and she counters "Well, think about it this way, if you're only my half brother, it's only half as bad."