Complex sentence: Francesco betrothed Anna in 1578 to Charles, Margrave of Burgau, son of Ferdinand II, Archduke of Austria (a brother of her mother), a portrait of Anna was sent to the Archduke.
Split into simpler sentences: Francesco betrothed Anna in 1578 to Charles, Margrave of Burgau, son of Ferdinand II, Archduke of Austria (a brother of her mother). A portrait of Anna was sent to the Archduke.