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In page Philip the Handsome:

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Philip's sister Margaret married John, Prince of Asturias, the only son of Ferdinand and Isabella and heir apparent to the unified crowns of Castile and Aragon.[4] The double alliance was never intended to let the Spanish kingdoms fall under Habsburg control. At the time of her marriage to Philip, Joanna was third in line to the throne, with John and their sister Isabella married and hopeful of progeny.[citation needed]