New York- a magical city

Nathan fairchild is the older brother of clary fairchild, he is 25 years old and clary is becoming 18. he's been training her since she was 14, when she was 15 he told her about the shadow world as he knew she's going to be a shadowhunter.
jocelyn originally planned to tell clary on her 18th birthday not knowing Nathan already did.

They both sometimes go out at night killing demons, Nathan was turned into a werwolf by the old alpha of the New Yorker pack when he was 20 and he became a shadowhunter when he was only 18. He was at the londoner institute for 2 years before returning to New York. In these two years Nathan became the parapatai of his best friend caroline Moore.
he told her about the whole Wolf turning thing and she ubderstood and never told anyone.

What if on clary's 18th birthday both fairchild siblings ran into shadowhunters of the New Yorker institute? What if Nathan didn't tell clary that he knew other shadowhunters?