The Cahills aren't your typical family. Most of its members don't even carry the Cahill name. But the common blood runs back for generations.
Crest Isn't Just a Toothpaste
The family crest is quite the paradox. It shows not just what unites the Cahills, but also what's keeping them apart.
The family as we know it today dates back to four siblings, each represented by a different point on the Cahill crest: Lucian, Ekaterina, Tomas, or Janus. Every Cahill is descended from one of these siblings, whether they carry the same name or not. Each branch is also represented by an animal, much like the houses of Hogwarts in Harry Potter.
And here's the problem. Just like the houses of Hogwarts competing in Quidditch, the branches of the Cahill clan love to compete against one another. Scratch that. They don't just love to compete, they love to hate each other. When Amy and Dan find a portrait of the four founding Cahills, they notice that "these guys were siblings who'd spent years annoying each other" (17.56). And their ancestors have spent centuries doing the same.
Crispy Chicken McMansion
One thing brings the family together, if only temporarily: Grace's funeral. Over 400 Cahills descend on her mansion for her funeral. The unity is short-lived when it burns to the ground. We find out later that the Holts cowardly set it ablaze to try to rid themselves of Amy and Dan, who watch in horror as "one stone tower collapses. The beautiful stained glass windows melted. The family crest […] crashed down and shattered on the pavement" (5.105).
To them, the house (and the crest) represented both Grace, their loving grandmother, and family. Now, thanks to Grace's clue hunt, the family is fractured more than ever, actively scheming against one another to get the upper hand. Watching the house literally crumble before their eyes only solidifies their realization that their family might not ever get along.