Question: mortal instruments city of bones is jace clary's brother

Answer: No, In the end it is revealed that he is actually Stephen Herondale and Celine Herondale's son and that Clary is not related to him. He and Clary have a conversation at the end of City of Glass in which he confesses that he doesn't know who he is, referring to his last name. Clary confirms that he is a Lightwood, and Jace accepts that, correcting people throughout City of Fallen Angels when they referred to him as ``Jace Herondale''. At the end of City of Heavenly Fire, Jace chooses to take on the Herondale name. Jace is also related to Will Herondale former leader of the London Institute and husband of Warlock Tessa Grey.