The 39 Clues

Beyond the Grave

                 by Jude Watson

Dan and Amy search on for their next clue, are almost massacred in a huge pink bubble and rescued by a hummus-loving Siamese. The children meet a lot of relatives, not necessarily friendly, and get to meet Grace's best friend from school - who manages to betray them too.
Basically, I get the idea from these books that the whole idea is:
"Hey, that guy looks trustworthy!" They get friends with him. "Oops, he just tried to kill me."
Amy and Dan are afraid of becoming Cahills themselves - they realize they're becoming more and more like their scheming family - suspicious and unfriendly.