Uncle Alistair might be the most deceitful Cahill we've seen so far. He double-crosses Dan and Amy not once, but twice. He allies with some of the biggest enemies, then double-crosses them, too. He kind of dresses like a pimp with a "diamond-tipped walking stick" (2.72), and he even lies about inventing the microwaveable burrito. Is nothing sacred?
What makes Alistair the untrustworthiest of all the Cahills is the fact that he seems so trustworthy in the first place. Are you with us on this one? He goes to great lengths to prove his loyalty to Amy and Dan. He offers them secrets, money, and even gets beaten up by the Holts to stall them. But his loyalty is only temporary. It's always just a few chapters before he turns on them to gain an advantage for himself.
We have to wonder why Amy and Dan ever trusted him in the first place. They don't trust Ian and Natalie, who aren't half as devious as Alistair is. Perhaps it's because Alistair is an adult, and Amy and Dan are unconsciously searching for adults to protect them after the loss of their parents.