On August 8, 1503, Margaret Tudor of England married James IV of Scotland. This marriage, of course, was one of the three Henry VII negotiated to forge alliances. Margaret married James IV; Mary, King Louis XII of France, and Arthur, Catherine of Aragon, daughter of Ferdinand II of Aragon and Isabella of Castile, the Catholic Monarchs of Spain.
The marriage of Margaret Tudor and James IV was blessed with a male heir, who reigned in Scotland as James V. By James V's second marriage, to Mary of Guise, was born Mary, who reigned briefly (as had Mary Tudor) as Queen Consort of France before returning to Scotland as Queen.
Both Margaret and Mary Tudor became widows: James IV died on the battlefield of Flodden, a battle overseen by Catherine of Aragon as Henry VIII's regent. Louis XII was 53 years old when he married the young Mary Tudor, hoping to conceive a male heir; after three months of trying, he died. Much against her brother's will, Mary got married to Charles Brandon, Henry VIII's good friend.
Margaret also remarried, to Archibald Douglas, 6th Earl of Angus and their daughter Margaret was born in 1515. Margaret Douglas, Countess of Lennox was the mother of Henry Stuart, Lord Darnley who would marry the Queen of Scotland in 1565. Margaret Douglas and Mary Tudor (Henry VIII's daughter, not his sister) were very good friends.
Although Henry VIII had been opposed to his younger sister's marriage to Charles Brandon, the Duke of Suffolk, he chose her heirs to succeed his children, bypassing his older sister's. The irony is that the available heirs of either Mary or Margaret were female: Frances Brandon who married Henry Grey and had three daughters: Jane, Catherine, and Mary; and of course, Mary, Queen of Scots!
Confused yet? Margaret and Mary, Mary and Mary, Margaret, Mary, and Mary! That's why I provide a bookmark guide to all the common names when I do book signings. The Tudor family tree is quite complex.
If you want to understand Henry VIII's relationship with his two sisters, I'd recommend Maria Perry's The Sisters of Henry VIII: The Tumultuous Lives of Margaret of Scotland and Mary of France. Tumultuous is an apt word!