Question: are jace and clary really brother and sister in shadowhunters?
Answer: Clary learns that Valentine Morgenstern, who is supposed to be long dead but truly is not, the main antagonist of the series, is her biological father and her mother's husband. At the end of City of Bones, Valentine tells them that Clary and Jace are siblings — which, as they discover later in the series, is a lie.