Philip and Carlos did not like one another, but suppose that despite that whole ordeal, Philip's health begins to deteriorate for unknown reasons, then completely fails him when Elisabeth dies during miscarriage. This leaves Isabel Clara Eugenia and Catalina Micaela as his surviving children, Isabel as the child queen regnant of the vast Spanish Empire, and both sisters as heiresses.
I'm sure one of them marries one of their Austrian Habsburg relatives as soon as possible, if not both of them, though Sebastian of Portugal is also young himself and could stand to gain the entire peninsula under the House of Aviz. Meanwhile, the 80 Years War had just gotten underway and the Spanish are still in a desperate war against the Ottomans for Mediterranean supremacy.
I'm sure one of them marries one of their Austrian Habsburg relatives as soon as possible, if not both of them, though Sebastian of Portugal is also young himself and could stand to gain the entire peninsula under the House of Aviz. Meanwhile, the 80 Years War had just gotten underway and the Spanish are still in a desperate war against the Ottomans for Mediterranean supremacy.