This inspired by and a vague memory of something seen on usenet a few months back):

Giulio Pignatelli, Prince of Noia, lived from 1587 to 1658.

He married his first wife, Zenobia, in 1601, and their son Fabrizio, duke of Monteleone and Prince of Noia, lived from 1604 to 1664.

Fabrizio Pignatelli in due course married his distant cousin Girolama in 1615 when he would have been 11 and she 14. They had a son, Ettore, also in due course duke of Monteleone and Prince of Noia, five years later in 1620. He lived to 1674.

In 1639, when he was 19, Ettore Pignatelli married his second cousin Giovanna, also known as Juana, heiress of vast conquistador estates in Mexico. They had one son, Andrea Fabrizio, in due course duke of Monteleone, duke of Terranova and Prince of Noia, born in 1640, and three daughters.

Andrea Fabrizio Pignatelli got married to another distant cousin in 1665, aged 25. They had just one child, a daughter, Giovanna, born in 1666.

Going back to Giulio Pignatelli, her great-great-grandfather: what happened to his first wife Zenobia is not recorded, but Giulio married his second wife Beatrice in 1638, when he was 61 and she was 28. Their son Nicola was born in 1648, when his father was 71, his mother 38, his half-brother Fabrizio 44, his nephew Ettore 28, and his great-nephew Andrea Fabrizio 8 years old, with his great-great-niece Giovanna’s birth 18 years in the future.

Andrea Fabrizio Pignatelli died in 1677, aged only 37, with just Giovanna as his heir to the vast estates on both sides of the Atlantic he had inherited. The interesting genealogical oddity is that Giovanna married her great-great-uncle Nicola Pignatelli in 1679, when she was 13 and he 31. They had three children over the next ten years (starting in 1682); she died aged 56 in 1723, and he died age 71 in 1730. Their descendants include the hereditary princes of Liechtenstein, also Sophie von Hohenlohe (assassinated with her husband, Archduke Franz Ferdinand, in Sarajevo in 1914), and large chunks of the German and Italian nobility.

On first sight it looks as if Nicola was Giovanna’s guardian as the nearest surviving male relative, and he married her as soon as it was legally possible to do so (possible a little sooner). But the truth may be more complicated. I should like to know more.