I guessed that had to be it (I think Peter had a passing mention of this hypothesis at one point) but it was irritatingly vague, especially since the children seemed to be reversed from how they ought to be -- if the children were switched as babies, then the child with the fairy mother was the one that was raised as Nicole and appeared to be human in every way, and the one with the human mother was the one with fairy powers. Which isn't a bad idea (nature over nurture, etc) except that we never got to know either of them as people, and never even saw Nicole except very briefly in the hostage-exchange scene.

That part of the story felt terribly underdeveloped, which wouldn't have been so weird if it were a minor subplot, but it was the central part of the plot. And the ending felt so rushed. I could've done with another big chunk of plot involving fairies at the end, and a lot less of Peter and Dominic wandering around in cow pastures and sitting in the police canteen.