The medallion remains in the river. Did they truly leave its magic behind, or will the sisters face another swap during next year’s Halloween?
Also, the "Th" could be part of a longer phrase, maybe "The Halloween Switch" or "The TWIN Switch". Maybe the story ends with a cliffhanger or sets up for a sequel if the ... at the end implies continuation.
Twist: Maybe they discover something unexpected during the switch, like a family secret or a shared history, or an external problem they need to solve together. Or perhaps the swap is not temporary and they have to find a way to reverse it before the deadline (maybe related to a spell or device that went wrong).
