Cautionary advice is important. Emphasize that these are fictional and shouldn't be taken as endorsements of real-life relationships that break societal norms.
I should provide examples of such stories. If there are specific novels or books, include them. If not, describe the general structure—how they might start with a family setting, introduce tension through relationships, and resolve with either a happy or bittersweet ending.
Next, common tropes. Forbidden love between siblings is a strong one, but there's always the taboo aspect. I should note that it's a fictional trope and not to be confused with real-life relationships. Other tropes could include family secrets, sibling rivalries with a romantic twist, etc.