In the original Mortal Instruments book series, Jace and Clary learn they are not actually siblings in City of Glass , once Jocelyn finally wakes up and explains the truth about their parentage; the key confirmation comes around chapter 15, and by chapter 20 they fully understand they are not brother and sister.

Quick Scoop

  • The “we’re not really siblings” reveal happens in City of Glass , the third book in the series.
  • Jocelyn’s explanation of Clary’s real brother (Sebastian/Jonathan) and Jace’s true origins untangles the fake sibling twist.
  • By late in the book (around chapter 20), Jace and Clary both firmly know they are not related and the sibling lie is over.

In-Universe Moment

Within the story’s timeline:

  • Valentine originally claims both Jace and Clary are his children, which makes everyone believe they are brother and sister.
  • After Jocelyn wakes up in City of Glass , she clarifies that Jace is not her son and that Clary’s real brother is Sebastian, not Jace, clearing the way for Clary and Jace’s relationship.

Why Fans Ask This

Many forum and Q&A discussions focus on:

  • Which exact chapter finally ends the “forbidden sibling romance” tension (most readers point to City of Glass as the definitive resolution).
  • How long it feels like the “are they siblings?” plotline drags across the first three books, which is a frequent topic in fandom debates and reread threads.

Information gathered from public forums or data available on the internet and portrayed here.