The title is misleading--the authors suggest but can not definitively state who betrayed the Franks.
Although they make a case for Bep's sister Nelly, they admit it is circumstantial and based on events and overheard conversations and memories that could be merely coincidences.

After making a case for Nelly, they then attempt to excuse her betrayal and the family's cover up. In the end, there just wasn't much evidence.

What the book accomplishes best is illustrate the long-lasting effects of war on individuals and families. Those effects were heightened in the occupied countries after the war ended when the survivors had to reconcile what they did to survive with their own morals and the judgement of others. Bep needed help.

I applaud the authors' dedication and research, but there just isn't much left to identify the betrayer confidently.