"If a father and his son take a widow and her daughter in marriage, so that the son marries the mother and the father the daughter, say, please, what is the relationship between the sons they will bear?"
Alcuin (?), Propositiones ad acuendos juvenes ("Problems for sharpening the young"), c. 800I would say that the relationship between the sons will be... awkward. Medieval bragging rights if you figure it out; I'll post the answer tomorrow.
Berlin Staatsbibliothek, MS lat. fol. 295, f. 80v (12th c.)