Isabel Neville, Duchess of Clarence
Lady Isabel Neville was the elder daughter of Richard Neville, 16th Earl of Warwick , and Anne de Beauchamp, 16th Countess of Warwick. She was the wife of George Plantagenet, 1st Duke of Clarence. She was also the elder sister of Anne Neville, who was Princess of Wales, by her first marriage and Queen consort of England by her second.
Isabel Neville was born at Warwick Castle, the seat of the Earls of Warwick. In 1469, her ambitious father betrothed her to Englands heir presumptive, George Plantagenet, 1st Duke of Clarence, the brother of both King Edward IV and Richard, Duke of Gloucester . The king opposed the marriage as it would bring the already powerful Earl of Warwick too close to the throne. The ceremony however took place in secret at Calais onJuly 1469, conducted by Isabel Nevilles Uncle George Neville, archbishop of York. Following their marriage Clarence joined forces with Warwick and traitorously allied with the Lancastrians led by Margaret of Anjou, queen consort to Henry VI. After Isabel Nevilles sister Anne was married to Edward of Westminster, Prince of Wales, the son and heir of Henry VI, Clarence rejoined his brother, realizing that it was now unlikely that he would become king.
Source: Wikipedia