Luke Garroway: There’s more to what happened at Renwick’s. Turns out Valentine is Jace’s father as well as Clary’s. They’re brother and sister.
Simon Lewis: Uh… brother and sis… Okay. Wait, so… Clary and Jace are… brother and sister. That means… That means they can’t… Well, what do you know?
From Shadowhunters – Season 1 Episode 11: ‘Blood Calls to Blood’ (1×11)