They are Madrigals. It comes from Grace and Hope's side of the family. Amy and Dan learn this in Viper's Nest, Book7.
Madrigals
Amy and Dan are part of the Cahill family, a secret society in The 39 Clues series. They belong to the Madrigal branch of the Cahill family, one of the most secretive and enigmatic branches.
In Book Seven: The Viper's Nest: Amy and Dan Cahill figure out they are Madrigals
i believe Amy and Dan are Madrigals.
Their branch is....... madrigal (srry i wanted it to be dramatic) :D
In the book THE VIPER'S NEST it reveals the Dan and Amy are Madrigals.
Amy and Dan Cahill are part of the Ekaterina branch in the 39 Clues series. The Ekaterina branch is known for their intelligence and passion for knowledge.
They are Madrigals. So are Grace and Hope Cahill.
Amy and dan are madrigals, the man in black is their great-uncle, fiske cahill, Nallie is a madrigal too, and Fiske and nellie are the new legal guardians of Amy and dan.
Amy and Dan are Madrigals. The Madrigals decended from Madeline Cahill. Olivia Cahill (the original cahill) was pregnant with her when Olivia died. Madeline then decided to carry out her mother's wishes. Madrigals try to keep the other branches from finding all the 39 clues. Hope this helps! :)
Lucian, well that is Madrigal, a type of Lucian.
Amy and Dan are siblings. Amy loves Dan as a sibling and a brother.