Complex sentence: Charles had previously been intended to marry the Princesse de Condé, and was emotionally close to his other sister-in-law, Marie Antoinette of Austria; his marriage to Marie-Thérèse failed to interest either bride or groom.
Split into simpler sentences: Charles had previously been intended to marry the Princesse de Condé. His marriage to Marie-Thérèse failed to interest either bride or groom.