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Jacob Herondale is Jace's twin..they were seperated at birth and while Jace grew up a shadowhunter, Jake was brought up as a mundane. He does however have the sight to see beyond the glimmer, unlike most mundanes; after all he does have the shadowhunter blood running through his veins...
Once Jake turned 17, he asked his foster parents about his real family and where they would possibly be. The only thing they told him was "Institute run by the Lightwoods in New York" After hours of pleading to allow him to go meet his family, they finally agreed and he packed his bags to leave in the morning..
After a couple days traveling, Jake reached New York and upon seeing the first person there, he asked where the Institute was. (Lucky for him it happened to be the mundane, Simon he ran into...as anyone else would think he was crazy) The boy was glad to help out and took him there...