What Tomorrow Brings, by IAmAllYetNotAtAll

Magnus finds young Alec and Isabelle living on the streets of New York in a time of chaos among the Downworlders and takes them in. He doesn’t know their last name, how they ended up on the streets, or why they seem to have so many secrets, but they have the Sight and he can’t bear to leave them on their own. 

As they grow older and discover the atrocities happening in the Shadoworld, Alec takes on the mantle his mother had left behind to protect his family. And Jace? He’s a whirlwind of flirting and secrets that crashes into their lives and refuses to leave. Together they may bring about the change the Shadoworld needs.

Teen, canon-divergence. This is simply wonderful – an epic-length story that’s about family and Shadow world politics as much as about two boys falling in love.