Infernal Devices Fanfic
Tessa has a secret half brother that she didn't know about
From her mother, who had a secret affair before Tessa was born. Her father forced her mother to put the baby up for adoption.
Brother is older than Tessa, shows up at the Institute seeking refuge
A demon wants him for his special shadowhunter blood
He found out that he was a shadowhunter because of this demon, and knows that Tessa is his half sister
Tessa has no idea
Set in late 1800s, after Jem becomes a Silent Brother and Tessa is with Will. She is carrying her first child.