'Where your sister at' is a pejorative phrase, synonymous with cruder insults such as 'screw you' or 'f**k you'. It is a derivative of the family of cusses which involve a subject’s close female relatives, typically mother or sister. It is widely believed to have spread through a piece of badly translated Spanish graffiti close to a skate park, when local and visiting skaters adopted it as a comedic insult. Its inferred meaning is that you should stop causing trouble and instead do something about the fact that your sister is in a place of ill repute copulating wildly with people of an unsuitable nature.
Skater 1: Perhaps you can’t pull ‘cause you dance like you’re at a Billy Ray-Cyrus concert
Skater 2: Yeah, well – where your sister at skate-tard