In Riddles in Mathematics (1961), Eugene Northrop proposes that two men can be each other’s uncle and nephew:

Mr. and Mrs. Allen … had a son Tom, and Mr. and Mrs. Black … had a son Dick. Mr. Allen and Mr. Black both died. And Tom and Dick, after they were grown men, each married the other’s mother. Dick and Mrs. Allen then had a son Harry, and Tom and Mrs. Black a son George. Now consider the relationship between Harry and George. Since Harry is the brother of Tom, George’s father, Harry must be George’s uncle. On the other hand George is the brother of Harry’s father, Dick, so Harry must be George’s nephew. In exactly the same way George is Harry’s uncle and nephew.

In Fun for the Family (1939), Jerome S. Meyer observes that if you marry the mother of your father’s second wife and have a son, and if your stepmother also has a son, then you can dine alone and still enjoy the company of your stepbrother’s nephew’s father, your father’s mother-in-law’s husband, and your stepmother’s father-in-law. Lewis Carroll considered a similar dinner.