No, if Sean Cassidy is the spelling of the person of whom you are inquiring. Nevertheless, Shaun Cassidy -- who portrayed Joe Hardy on "The Hardy Boys" in the 1970s TV series. Then, yes, David and Shaun Cassidy are half brothers, they have different mothers. Their father was Jack Cassidy.
Shaun Cassidy -- half brother to David Cassidy is not a dancer. If Sean Cassidy is a different person, then, 1) he is not related to David Cassidy, 2) you have a photo of David Cassidy's half brother, Shaun here.