Because the Carstairs owe the Herondales.

“I asked in the Spiral Labyrinth as well,” said Tessa. “There was never any word. Until suddenly we heard that someone was telling stories to the children of this Market, stories of love and revenge and misery. We heard a whisper of the name Herondale.”

She said the name that had once been hers very softly. Alec jumped as if someone had shouted it in his ear.

Neither Jem nor Tessa mentioned Catarina Loss, who had carried the first lost Herondale child over the seas and raised him on strange shores. That wasn’t their secret to tell. Jem trusted Alec, but he was still a Shadowhunter, and his father was the Inquisitor. Jem and Tessa were both well aware of the sentence the Law would pass on Catarina for her act of love and mercy.

“I’ll ask Juliette,” said Alec. “I’ll find out whatever I can. I won’t go home until I’ve helped you.”

“Thank you,” Jem said.

“Now Rafael has to go to bed,” said Alec.

“We have a nice little room for you,” Tessa told Rafael in Spanish, her voice soft and encouraging. Rafael shook his head. “Do you not want to be alone?” Tessa asked. “That’s fine too. You can sleep with me and Jem.”

When Tessa reached out her hands, Rafael turned his face into Alec’s bicep and screamed. Tessa drew back at the long mutinous howl. Alec automatically put his arm around the child.

“Lily’s vulnerable during the day,” he said. “I’d rather stay with her. Will you be all right in the windowless room, Rafael?”

Lily translated. Rafael nodded emphatically.

Jem showed them the way. At the door, he caught Alec’s arm before he could follow Lily and Rafael.

“I appreciate this,” said Jem. “I truly do. Please don’t tell Jace yet.”

Jem still thought about Jace, that fierce helpless child he’d met on a dark sea, and the young man burning with heavenly fire. He’d imagined a hundred scenarios where he did better by Jace. If he’d been the Silent Brother who cared for Jace after his father left him, if he’d spent more time with Jace, if Jace had been just slightly older, the age Will had been when Jem first met him … maybe Jem would have known.

But what could he have done for Jace, even if he had known?

“I don’t want Jace to think he has family somewhere he won’t get to know,” said Jem. “Blood is not love, but it offers a chance for love. He never had the chance to know Céline Montclaire or Stephen Herondale. I don’t want him to feel he is missing another chance.”

Jace was happy in New York, though Jem had not helped him be so. He had his love and his parabatai and his Institute. If Jem couldn’t help him, at least he did not want to hurt him.