In the book THE VIPER'S NEST it reveals the Dan and Amy are Madrigals.
In Book Seven: The Viper's Nest: Amy and Dan Cahill figure out they are Madrigals
i believe Amy and Dan 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.
Madrigals
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 siblings. Amy loves Dan as a sibling and a brother.
Lucian, well that is Madrigal, a type of Lucian.
They are Madrigals descended from Madeline Cahill through Grace and Hope Cahill.
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.
madrigals made by madeline cahill the last branch of the cahils
They are Madrigals. It comes from Grace and Hope's side of the family. Amy and Dan learn this in Viper's Nest, Book7.