Prince Nicola Pignatelli (1648–1730) married his agnatic half-great-grandniece, Princess Giovanna Pignatelli (1666–1723).
Nicola was the son of Giulio, Prince of Noia (1587-1658) via his third wife, Giovanna was a great-great-granddaughter of the Prince of Noia via his first wife.
Giovanna’s father was born in 1640 which means that Nicola was a (half-) granduncle already at birth.
Count Lennart of Wisborg (1909–2004; formerly Prince of Sweden and Duke of Smolandia) was already a great-grandfather before his last child via his second marriage was born in 1982. This daughter, Countess Diana of Wisborg, was born a (half-) grandaunt.
I know that age and generation do not always correlate. Crown Prince Rupprecht of Bavaria, for instance, fathered his last child at the age of 66, by which time he was already a grandfather. His son Albrecht (the only surviving child from his first marriage) was by then 30 years old and a married father of three children of his own, at the time. What this means is that Duke Franz, present head of the royal house of Wittelsbach, has an aunt younger than himnself.
Of course, Princess Sophie is only a half-aunt, since she was born to her father's second marriage. Still, she technically belongs to the same generation as that of the late Duke Albrecht.
It's a rare occurrence, to be sure, but not entirely unheard of: I believe there once was a Spanish princess who married her own nephew, who was actually two years her senior. Does anybody know who exactly she was?
Can anybody name other examples in European royal history, where a person had an aunt/uncle younger than himself?
When one is talking of cousin relationships, it should be even an even more common occurrence. Princess May of Teck, for instance, technically belonged to the same generation as that of King Edward VII of Great Britain. After all, her mother (Princess Mary Adelaide of Cambridge) was a first cousin of Queen Victoria. This made the future Queen Mary (consort of King George V) into a second cousin to her father-in-law. Yet, she was two years her husband's junior. But that was no surprise: Princess Mary Adelaide, in addition to being 14 years younger than Queen Victoria (who married and had children young), was married at the relatively late age of 33. By 1867, when she became a mother for the first time, her sovereign cousin was already a grandmother several times over.
This situation was similar to the late King Michael and Queen Anne of Romania, who were second cousins, once removed. The wife was a second cousin to her mother-in-law (Princess Helen of Greece and Denmark), who like herself was a great-granddaughter of King Christian IX of Denmark. But Anne (who was two years her husband's junior) descended from the Danish king through his youngest child, Prince Valdemar, who fathered her mother (Princess Margarethe) in 1895, a year before Queen Helen of Romania was born.
Anyhow, for practical reasons, I wanted to focus only on aunts and uncles, nieces and nephews. What's the list of such marriages?