How we cite our quotes: (Chapter.Paragraph)
Quote #1
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.
Quote #2
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.
Quote #3
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.