Too bad for them that Pop Pop decides to prove that being crazy isn’t the same thing as being stupid. Aware that the ruse is up, he fills in the backstory behind the twist. Prior to taking up residence in the farmhouse, he and “Nana” were patients at a nearby mental hospital where the real Nana and Pop Pop were regular volunteers.
Jealous at their happiness over the impending visit of their grandkids, the frauds forced their way into the home and murdered the couple with a hammer, stashing their bodies in the basement. (Throughout the movie, other folks from the hospital have been stopping by the house to check up on the popular duo, but their replacements have conspired to be “out for a walk” during these visits.) They then went to the train station to pick up Becca and Tyler who were none the wiser because, remember, no pictures! Also, no Mom around to warn them otherwise. (Not for nothing, but this twist really does elevate Hahn to Worst Mother in the World status.)