I'm not human, I'm a Mikaelson
What if stiles wasn't human? in fact he hadn't been human for over 1000 years. What is he is really Genim "Stiles" Mikaelson?
Then add his little sister into the mix. Mikayla Mikaelson-Marshall, friendly, smart, kind, loving, happy, innocent and most importantly the only Mikaelson who isn't broken, or so everyone thinks.
Underneath it all she is more broken then everyone else but she puts on a happy facade but in reality she is right on the edge and if anything else happens she might just go off the rails and turn her humanity off.
And we all know a Mikaelson without humanity isn't good, even if it is the innocent one.
A TVD/TO, TW and a bit of shadowhunters cross over.