Complex sentence: Joanna was supported by Portugal, while the eventual winner, Henry's half-sister Isabella I of Castile had the support of Aragon because of her marriage to Ferdinand II of Aragon, and later in the war, France also supported her right to the throne.
Split into simpler sentences: Joanna was supported by Portugal while the eventual winner, Henry's half-sister Isabella I of Castile, because of her marriage to Ferdinand II of Aragon, had the support of Aragon. Later in the war, France also supported her right to the throne.