Welll, my train of thought was that we only know what Arthur told us, and he's a serial killer. He's crazy. He broke his son's finger because he smashed the car. So I don't think it's out of the realm of plausibility that maybe Vera's still alive, and he just made up the story.
There's no reason to trust what he says. Who knows whose ashes are in the urn. Perhaps Vera felt ashamed of what they'd done, and rejected Arthur and he killed her in the bathtub. He killed his father, he might've pushed his mother off a building.
I have to assume the twist concerns Arthur and his family. Christine mentioned other children. Arthur must be in his 60s, his wife looks 40s, their kids are teenagers. He must have other children all across the country.
