Question: is jace really clary's brother in the books?
Answer: At last, Magnus wakes Clary's mother Jocelyn, who informs Clary that Jace is not her brother. Her real brother is Jonathan Morgenstern, who has, this whole time, pretended to be Sebastian. It is soon revealed that Sebastian is a spy for Valentine and that he tried to seduce Clary into teaming up with him.