Manuel I of Portugal
Manuel's mother was the granddaughter of King John I of Portugal, whereas his father was the second surviving son of King Edward of Portugal and the younger brother of King Afonso V of Portugal. In 1495, Manuel succeeded his first cousin, King John II of Portugal, who was also his brother-in-law, as husband to Manuel's sister, Eleanor of Viseu.
Joanna of Austria, Princess of Portugal
The marriage was cut short when John Manuel died of tuberculosis at age sixteen on January 2, 1554. However, Joanna was pregnant by that time, and the future Portuguese king Sebastián I was born on January 20, 1554.