Tudor sibling marriage

In 1528, King Henry VIII was in the midst of a crisis. His desire for a legitimate male heir led to him asking the Pope for a divorce from his current wife Catherine of Aragon, in order to marry Anne Boleyn. Cardinal Campeggio was sent as Papal Legate to England in order to examine the case and see what resolution could be made.

Cardinal Campeggio

Cardinal Campeggio arrived in England on the 8th October 1528. He had been instructed by the Pope to secure an outcome which would not include a divorce; he had tried to persuade the King to reconcile with his wife, and when this failed he urged Queen Catherine to enter a convent. A few weeks later, he wrote to Giovanni Baptista Sanga who was a papal advisor that;

"They have thought of marrying the Princess, by dispensation from his Holiness, to the King's natural son, if it can be done. At first I myself had thought of this as a means of establishing the succession, but I do not believe that this design would suffice to satisfy the King's desires."

This suggests a marriage between Princess Mary, then aged 12, and her illegitimate half-brother Henry FitzRoy, then aged 9. This marriage, despite being incestuous, would allow King Henry to pass the throne on to his son as well as his daughter; thereby maintaining the Tudor bloodline.
It is unknown who thought up this proposal, however the Pope agreed to provide the required dispensation for this marriage to go ahead. This permission however what dependent upon the condition that King Henry stops requesting a divorce from his wife. It was this condition which led to King Henry losing interest in the plan.

Henry FitzRoy

Henry FitzRoy was at this time Henry's only acknowledged son, illegitimate as he was. He had been greatly favoured by his father the king his whole life, often being placed before the Princess Mary. With his lack of a legitimate male heir, Henry had often considered passing a law declaring Henry FitzRoy as his heir. In 1536 this was almost certain after King Henry's separation from Anne Boleyn and the declaration that both his daughters Mary and Elizabeth were illegitimate - the same legal status as Henry FitzRoy. By 1536 FitzRoy was married to Mary Howard, had experience leading an army as well as handling royal duties such as Parliament meetings. However, he died suddenly that year before any action was formalised.