Oooo remembered a good example from medieval history.
King Louis VII’s third wife was Adele of Blois, with whom he had his only son, Philippe Auguste, as well as one daughter, Agnes. From his first marriage to Eleanor of Aquitaine, Louis already had two daughters named Marie and Alix. From his second marriage to Constanza of Castile, he’d had two daughters named Alais and Marguerite. With me so far?
Now, Adele of Blois had a pack of siblings, but the important two were Henri of Champagne and Thibaut of Blois. It was arranged that Henri and Thibaut should marry Louis VII’s daughters from his first marriage – Henri married Marie and Thibaut married Alix. So Louis’ brothers-in-law were now his sons-in-law, as well as brothers-in-law to each other.
Louis and Adele’s son, Philippe Auguste, married as his first wife Isabel of Hainaut. And Isabel’s brother Baldwin married Henri and Marie’s daughter, also named Marie. So Philippe’s brother-in-law was married to his niece, who was also his cousin. Confused yet?
But it gets weirder. Louis VII’s first wife, Eleanor of Aquitaine, had remarried to King Henry II of England and gave him many children. Their son Young Henry married Marguerite, Louis VII’s daughter by Constanza. And Young Henry’s brother Richard (of Lionheart fame) was betrothed to Marguerite’s full-sister Alais but broke off the betrothal because… wait for it… he accused his own father of schtupping his fiancee! Yes, allegedly Henry II cheated on his wife with the daughter of her own ex-husband, while said daughter was engaged to their son!
Not to mention there have been suspicions about the nature of the, ahem, relationship between Philippe Auguste and Richard the Lionhearted, and for that matter, Philippe’s relationship with Richard’s *other *brother, Geoffrey. Remember, Geoffrey and Richard and Henry’s mother had been married to and had children with Philippe and Marguerite and Alais’ father. All these people had nieces and nephews in common. It’s about as twisted as you can get without invoking actual sibling incest.