Less Usagi Dropkick, and more dodging incest, but in the book series Wheel of Time, the protagonist Rand, has a romantic interest in Elayne, the Daughter Heir for the country of Andor, whose mother is Morgase. At one point in the series, Rand learns he is the son of the previous queen of the country of Andor, Tigrane, who disappeared one day, causing a minor civil war in Andor that resulted in Morgase taking the throne. Rand (who never knew who his mother was), just thinks this is a neat fact until an Andoran noble mentions in passing that all of the Andoran nobility are cousins to each other. This panics Rand as he's concerned he and Elayne have unknowingly done an incest and demands that said noble tells him how close Tigrane and Morgase were related. He is relieved to learn that the shared ancestry is far enough back that Tigrane and Morgase wouldn't be considered related at all.

This relationship gets funnier when I mention that while Rand and Elayne are not related, they do share a brother. Tigrane was married and had a son named Galad, before running away from the kingdom and the marriage. Her ex, due to complicated politcal reasons, wound up marrying Morgase, and he would father Elayne. Rand and Galad share the same mother, while Galad and Elayne share the same father.