POP MOTIFS are a subtle way of adding resonance to a story, while keeping the story as immediate and accessible as possible.
The two half-sisters in "Kevin and Koreena" are an example of a pop motif. They show in shorthand the rich/poor gap in society. (Think "Cinderella.") They follow the plot motif of the mysterious family relation. (Think Dickens.) They key into classic Greek literature, where many of the heroes like Achilles and Hercules are half-god, and offstage gods and goddesses drop by every so often to influence the plot.
The pop motif sounds intrusive. It's not. Used, properly, it blends into the tale, its effect subliminal.