
Source
Widowed at the age of eighteen and a half, the Infanta Doña Juana (daughter of Charles V) was reputed to be one of the most beautiful and graceful women of all Castille; and after her marriage of barely two years duration with Don Juan of Portugal, on her return to Spain, and in absence of Philip II, she had not only taken charge of Don Carlos, but had conducted the affairs of the monarchy in a manner which had gained the esteem and admiration of her brother and his subjects.
Rejecting the advances of two suitors - the Duke of Ferrara and Francesco de Médicis -, it seemed as if the crown of the Queen of Spain alone would be capable of replacing the one she had lost by the death of the Infante of Portugal. The Cortés of Castille, in a solemn address to Philip, earnestly recommended the marriage, to which recommendation he had replied in favourable terms.
Historically, Don Carlos was not disposed to marry his aunt, or marry for his father's pleasure, but imagined himself in love with his cousin Archduchess Anna of Austria, grand-daughter of the Emperor. He pined for her, admired her portrait and declared he would never marry another.
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Say he had reacted positively and agreed to the marriage with his aunt. The Spanish realm would benefit from the steady hand of the stern Princess of Asturias, and Philip II would be at liberty to focus his attentions elsewhere. The Princess had conceived fairly quickly in Portugal, so she could provide Carlos with a nice number of heirs prior to his death (assuming he died, as historical, in 1568).
Would create an interesting situation with the very capable Juana ruling on behalf of her brother/father-in-law and later (if Carlos survives longer) madman nephew-husband. Furthermore, as she was the mother of King Sebastian of Portugal (whose heir was Don Carlos), the King of Portugal and the future King of Spain would be half-siblings, with the Spanish being heir to Portugal also.
What ramifications would this have - for Philip II (his own third marriage, foreign policy, etc), for the Portuguese succession, for the Spanish succession, for the government of Castille?
If there's enough potential, I might try a timeline out of this.
