On 5 February 1525 at Salamanca aged eighteen, she married her sister's stepson, Joao III, King of Portugal, and they became the parents of nine children of which only one son survived. This son died aged sixteen, leaving a pregnant wife. The baby's mother returned to Spain after the birth, leaving the baby with his grandparents, King Joao III and Queen Catharina. 
On 11 June 1557, only three years after the death of their last child, Catharina's husband died without leaving instructions about a regency. A hastily convened council of nobles declared that it had been his wish that Queen Catharina should undertake the office and she was duly appointed. She was almost as bigoted as her husband had been and was so aggressively Spanish that her rule became unpopular with all classes. 
It was widely believed that she was intriguing to secure the eventual succession of her nephew, Philip II of Spain. The Regent had a rival in the person of her brother-in-law Cardinal Enrique, who felt the regency should have been his. Catharina was not a bad ruler but apparently grew weary of it. She first voiced her intention to resign in favour of Cardinal Enrique in December 1560, but the plan was not put into effect and approved by the Cortes until two years later, when Enrique took over on the proviso that Sebastian, who was to remain in his grandmother's care, should come of age at fourteen.
 

Source: Leo van de Pas