Isabel Kabra killed Hope Cahill.
Isabel Kabra set the fire that killed them(Ian and Natalies mom)
Amy Hope Cahill
Hope Cahill and Arthur Trent
The Mandrigals area secret branch of the Cahill family. The Mandrigals are determined to keep the secret of eternal life safe. They killed Hope Cahill because she learned the secret. They also set fire to Grace Cahill's mansion to keep people from finding the secret library and more Clues. They currently are tracking Amy and Dan Cahill to stop them from finding the secret to eternal life. This is just a theory.
They are Madrigals descended from Madeline Cahill through Grace and Hope Cahill.
According to The 39 Clues: Vespers Rising, Jane Cahill was the youngest daughter of Olivia and Gideon Cahill at 10 years old in 1507, the year that the Cahill home was burned down, Gideon was killed, and the Cahill family was scattered across the globe. If you do the math, then, Jane Cahill would've been born in the year 1497.
They are Madrigals. So are Grace and Hope Cahill.
Grace Cahill killed Alister Ho's father in The 39 Clues.
Amy Cahill hid a clue in her arm in "The 39 Clues" series. She had a small microchip implanted under her skin which contained a vital clue to the location of the 39 Clues treasure.
Grace and Fiske Cahill's car crash was orchestrated by Isabel Kabra, a member of the Lucian branch of the Cahill family. She was under orders from her mother, Cora Wizard, who is a part of the Ekaterina branch.
Nellie Gomez hid a clue in her arm, specifically in the form of a microchip that contained information needed by the Cahill family in "The 39 Clues" book series.
They are Tim Cahill, Mile Jedinak, Harry Kewell, and Mark Schawzer.