It's not just a trick: it's the fact that he drank the blood of the dead girl. The girl is not human. She wakes up, and they let him possess her body. They're already there, and the girl is ready to give her body to them. Uh-oh. It turns out that the body belongs to their daughter, not to the other way around. The mother-in-law wants to know what's going on, so she tells the boy that she's always been his mother, and that he should stop following her around and asking her to pick him up. The boy says that he's more handsome than his father, and he wants to talk to his mother in the yard. He wants to be alone with her, so he tells her to stop following him around and not to touch his mother's body. He says that his mother could be happy with him as long as he was happy, and so he forgives his father. He's sure that they're going to meet again, and maybe they'll even become family.