Ah, that's a question that causes a lot of tension in the 'Shadowhunter Chronicles' by Cassandra Clare. In 'City of Bones', the first book of 'The Mortal Instruments' series, it's revealed that Clary Fray and Jace Lightwood are siblings.
But, don't fret! In the subsequent books, it's unveiled that it was all a deception played by Valentine Morgenstern, their supposed father. In a shocking twist of fate, Jace actually turns out to be the son of Stephen Herondale, making him and Clary not siblings after all. This rollercoaster sibling drama is a major plot device throughout the series, keeping readers on their toes.
So, in a nutshell, though Clary and Jace were initially believed to be siblings due to some cleverly crafted deception, they are not siblings in the true sense. What an emotional ride, isn't it?