Responsibility
"I'm just trying to be…"
"An older brother," said Isabelle. "I get it."
Izzy got to her feet, went across the room, and put her arms around Alec. Their dark hair, precisely the same color, mixed together as Isabelle whispered something into her brother's ear…
-City of Lost Souls, Chapter 1, page 26
Alec had always felt like it was his duty to protect his siblings, Isabelle knew that. She also knew that his self-confidence issues usually prevented him from actually fulfilling that duty. Regardless, she still loved him. He was her brother, the constant, and they've shared numerous secrets over the past few years.
Now that Jace is gone, Isabelle sees that her brother's found a new responsibility- taking care of Clary. It's not unreasonable, but sometimes, Izzy thinks, Alec worries too much.
This is one of those times. She knows the dangers, she knows he'll fret the entire time they're gone, but it's necessary. It's Jace that's in danger, and between the three of them, they'll burn down the world to get him back.
So Izzy goes over to her brother, winding her arm around his shoulders, and reassures him in the way only she can.
"I'll be okay," she whispers. "We'll all be. Jace especially. I know the faerie court, I know how they work-" (she doesn't need to remind him of her short-lived romance with one of them)- "and I'll bring Clary home safely. Trust me."
Alec doesn't entirely believe her still, so she presses a kiss to his cheek. "Trust me," she breathes.
He has no choice. If he doesn't trust his sister, who else can he trust in this cold, cold world? (Besides Magnus, of course. But then blood did always run thicker than water.)
So Alec wraps a finger around Izzy's jet black hair, and tugs it the way he used to when they were children. This time, she doesn't threaten to cut his finger off (lucky him). She smiles instead, and for a moment, the world shines brightly again.
"I'll have to tell Magnus," he says.