It's books like these that make me really worried about Eve and Roarke having kids at some point. Eve doesn't handle kids all that well. She's doing okay by the end, but it's a little shaky.
The Survivor in this book is a little girl named Nixie who loses her whole family when two men go into her house and kill everyone else inside. She lives because she was out of bed getting a drink from the fridge. She had a friend over who was also blonde, so the two men killed her friend, assuming that friend was really Nixie.
The investigation turns up nothing on the family that would suggest it's personal, but it can't be anything but given the evidence. The parents haven't had affairs, there's no money trouble and the kids are 'normal' and don't have any problems.
The murders come back to revenge. It really is that simple. The parents did there jobs and their whole family paid the price in the end.