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William the Conqueror and Tostig Godwinsson, Earl of Northumbria.

brothers-in-law. William the Conqueror and Tostig of Northumbria each married a daughter of Baldwin V, Count of Flanders. Tostig married the elder daughter, Judith, whereas William married the younger daughter, Matilda. Tostig sided with Harald Hardrada of Norway against his brother, Harold II of England, and was killed at the Battle of Stamford Bridge on September 25, 1066. Harald of Norway was also slain. A few weeks later, on October 14, 1066, William and Harold clashed at Hastings, where Harold II was killed.

Brian Boru, High King of Ireland and Sitric Silkbeard, King of Dublin.

step-father and step-son and father-in-law and son-in-law. This one is kind of weird. Brian was the third husband of Sitric's mother Gormlaith of Leinster (the first two having been Sitric's father, Olaf of Dublin, and Malachi Mor of Meath, High King of Ireland). However, what is not commonly known is that this was a double alliance. Sitric married one of Brian's daughters from his first marriage, Emer. Must have made for some interesting conversation during the Battle of Clontarf, don't you think? Interestingly, if the Norse had won that battle, Sitric had promised Gormlaith to both Sigurd of Orkney and Brogar of Man in order to gain their support against Brian.

John, King of England and Otto IV, Holy Roman Emperor.

uncle and nephew. John's sister, Matilda, married Henry 'the Lion', Duke of Saxony and Bavaria. Otto, their son, was elected Holy Roman Emperor in 1209, succeeding his father-in-law, Philip of Swabia.

Alexandra, Queen of England (wife of Edward VII) and Mariya, Empress of Russia (wife of Alexander III).

sisters. This brings to light one of the great misconceptions that I've found with relationships between the royals. Most sources infer, or down- right claim, that Nicholas II was Edward VII's nephew through blood descent from Victora. This is not true. The kinship comes from the Alexandras, one was Nicholas's aunt, the other his wife (who was Edward's niece, her mother having been one of his sisters).

Henri I, King of France and Yaroslav the Wise, Grand Prince (or Duke) of Kiev.

son-in-law and father-in-law. Yaroslav had three daughters: Anna, who wed Henri; Anastasia who married Andrew I, King of Hungary, and Elizabeth, who wed Harald Hardrada, King of Norway. Interestingly, both Henri and Harald opposed William of Normandy.

Catherine of Aragon, Queen of England and Francis I, King of France.

aunt-in-law and nephew-in-law. Francis's second wife was Catherine's niece, Eleanor of Austria, Dowager Queen of Portugal. Eleanor was the daughter of Catherine's sister, Juana la Loca, Queen of Castile. Ironically, Francis supported Henry VIII's marriage to Anne Boleyn. Whatever happened to family loyalty?

Edward the Confessor, King of England and St. Margaret, Queen of Scotland.

great-uncle and great-niece. Edward the Confessor was the youngest son of Ethelred the Redeless and Emma of Normandy. Emma, however, was Ethelred's second wife. His first spouse, Aligfu of Devon, gave him a son, Edmund Ironside. When Ethelred died (1016), Knut of Denmark and Edmund fought over England until they reached a stalemate, and divided the realm between them. But, then, Edmund died (1016), and Knut inherited the whole of England. Edmund, however, left two young sons who fled with their mother to Hungary. One of these sons, Edward the Exile, lived to adulthood (dying in 1057), and produced three children: St. Margaret (who married Malcolm III, King of Scotland), Edgar the Aetheling, and Christina.

Maud of England, Queen of Norway (wife of Haakon VII, King of Norway) and Margaret, Crown Princess of Sweden (first wife of the future Gustav VI Adolf, King of Sweden).

cousins. They were both descended from (who else?) Victoria, Queen of England. Maud was the daughter of Edward VII; Margaret the daughter of Arthur, Duke of Connaught, the third son (and seventh child) of Victoria and Albert.

David I, King of Scotland and Matilda of Boulogne, Queen of England.

uncle and niece. Don't confuse Matilda of Boulogne, daughter of Count Eustace III, with Matilda of England (a.k.a. Empress Matilda), daughter of Henry I. It's easy to do, I know. The kinship comes through Matilda's mother, Mary, who was David's sister. However, Empress Matilda's mother, Edith-Matilda, was also David's sister. To add to the confusion, Matilda of Boulogne married Stephen of Blois, who was also Empress Matilda's cousin, his mother having been Adela of England, Henry I's sister. During the war of succession that raged between Stephen and the Empress after the death of Henry, King David supported the Empress, losing badly at the Battle of the Standard.

Alexander II, King of Scotland and Llewelyn ap Iorwerth, Prince of North Wales.

brothers-in-law. Alexander II's first wife was Joan of England, daughter of King John and Isabella of Angouleme; Llewelyn's first wife was Joan Plantagenet, John's illegitimate daughter.