At last, Magnus wakes Clary's mother Jocelyn, who informs Clary that Jace is not her brother. In fact, Sebastian (the real Jonathan Morgenstern) is her brother. It is soon revealed that Sebastian is a spy for Valentine and that he tried to seduce Clary into teaming up with him. Clary then tries to locate Jace to help him. She runs into Valentine, who ties her down in order to complete his summoning of the Angel by using her blood. Then, when Jace arrives to rescue her, Valentine forces her to watch as he fatally stabs Jace in the chest. Upon summoning the angel, Clary changes the marks on Valentine's summoning circle, giving her the ability to control the Angel, and after Valentine is slain, she wishes to have Jace back. After Jace is brought back to life, the two get together, now that they know that their love for each other isn't incestuous.

Is it true that in the mortal instruments are clary and jace related?
no