Jace and Clary are not siblings in "The Mortal Instruments". Jace's father, Valentine, convinces Jace and Clary that they are siblings to keep them apart. They spend ample time avoiding each other after this revelation and are disgusted that they had been attracted to each other.
"The Mortal Instruments" is a series of six books written by Cassandra Clare. "City of Bones" is the first novel in the series in which Jace and Clary are introduced. "The Mortal Instruments" series has been translated into 34 languages, including Bulgarian, Chinese and Hebrew, with the tragic relationship between Jace and Clary among the most popular of storylines in the series.