Dan Cahill thought he had the most annoying big sister on the planet. And that was before she set fire to two million dollars. (2.1)
Dan might think Amy's annoying, but it shows how close they are when he doesn't get mad at her at all for giving up their two million dollar inheritance. They are not rich kids, but they put each other's ambitions ahead of money.
The mansion filled up with Chinese Cahills and British Cahills and South African Cahills and Venezuelan Cahills. (2.32)
This is a clue (not one of the titular 39 clues, but a clue nonetheless) that the Cahill family is a lot larger than you might think. This isn't your typical family reunion; this is a family tree with branches on a global scale.
If you are in this room [...] you are a Cahill. Whether your surname is Cahill or not doesn't matter. Everyone here has Cahill blood. (2.113)
Here's another hint that anyone could be a Cahill. The subtle foreshadowing makes it not as surprising when we find out that Ben Franklin, among others, are Cahills.
Amy Cahill thought she had the most annoying little brother on the planet. And that was before he almost got her killed. (3.1)
Check out the tight bond between Dan and Amy. How many times has your brother or sister almost gotten you killed in an explosion? Talk about being in the trenches together.
Amy did something completely disgusting. She hugged him. (6.107)
In case you forget with all the action-packed globetrotting, Dan's eleven. And a boy. So getting hugged by a girl, especially his sister, is really gross. (But also very sweet, no?)
Amy felt a twinge of guilt leaving the Holts behind unconscious, but they had no choice. (14.117)
By this point, they've left a bunch of relatives behind: they left Alistair in the burning mansion, the blown-up Starlings, and now the Holts. We don't think they're that guilty after all, putting the competition ahead of family. That's kind of how the Cahill clan rolls.
Pretty soon it was just Nellie, Amy, and Dan, sitting in a corner of the platform like a homeless family. (16.129)
Dan and Amy are orphans, so they're having to form their own, non-traditional family. We're not sure what Nellie's home-life is like, but she's become part of Amy and Dan's family by taking care of them and being someone they can trust.
These guys were siblings who'd spent years annoying each other. (17.56)
This statement refers to the original four Cahills. Could it be that this family has been fighting amongst each other since the very beginning? That makes Amy and Dan, two siblings who get along, an exception.
Amy realized more than ever how alone [she and Dan] were in the world. All they had was each other. (18.55)
Being orphaned at a young age, Amy and Dan have had to rely on each other for a long time. Even though she just now realizes it here, she's been taking care of Dan—and he's been taking care of her—for ages. Here's hoping that doesn't change anytime soon.
[Dan] still didn't like the idea that Ben Franklin was related to Ian and Natalie. (19.12)
That's one thing about family—sometimes an awful family has an exceptional member or two.