What book do you find out that jace is not brother?

You're likely referring to the "City of Glass" (the third book in the "Mortal Instruments" series by Cassandra Clare) where it is revealed that Jace is not biologically related to the Lightwood family.

Here's what happens:

* Jace discovers his parentage: He learns that he was adopted by the Lightwoods as a child, and his biological father is actually Stephen Herondale, a famous Shadowhunter.

* His mother's identity: He also discovers his mother is Valentine Morgenstern, the main antagonist of the series.

* The significance of the revelation: This revelation causes immense emotional turmoil for Jace, particularly as he learns his true father was a close friend of the Lightwoods, and his mother is the source of the greatest threat to the Shadowhunter world.

This twist is a significant plot point that greatly affects Jace's character development throughout the series.