First of all, The Tudors actually combined the two sisters of King Henry VIII and made one sister - something I don't really understand, but I think they did it to avoid confusion. Henry had an older sister named Margaret, and a younger sister named Mary - well, in the show, they combine the two and name her Princess Margaret. Because of this, I think many people walked away from the show thinking it was Margaret who married Charles Brandon, but it wasn't! It was, in fact, Mary Tudor, who was thought to be the most beautiful princess in Christendom.
Second, in The Tudors, our Mary figure is shown to be forced into a marriage with the elderly King of Portugal. Although the King that she was thrown into marriage with was 52 years old (she was 18), he was actually Louis XII, King of France. Although I can't imagine her being thrilled about marrying such an old man, she did become the Queen Consort of France, which was a nice bonus.
With the King of France dead, Princess Mary was summoned back to England by her brother. It is widely believed that she was already in love with Charles Brandon, who had just been named the Duke of Suffolk, but that King Henry would never support a marriage between the two of them - Royal marriages were made to form alliances with countries and benefit the monarchs in some way, so a union with Brandon would not have helped King Henry at all. However, he sent Brandon to bring Mary back from France, and made him promise not to propose to her while he was there. Funny thing is, the two of them decided to marry in secret on March 3rd while still in France! This was an act of treason against the King, and they received a great deal of his anger when Cardinal Wolsey passed on this information.
Mary was Brandon's third wife, and she acted as stepmother to his two daughters, Anne and Mary, and raised them alongside her own four children by Brandon. Although she was now technically the Duchess of Suffolk, she continued to be known in England as "the French Queen" until she died in 1533.