In 926, Athelstan "gave" his sister to Sihtric, the Viking king of York. She was (according to William of Malmesbury) Athelstan's only full sibling - their mother, Ecgwynn, it was rumoured, being the concubine of their father, Edward 'the Elder', rather than his wife.
William of Malmesbury says he could find "no written record" of Athelstan's sister's name, however, Roger of Wendover says that Athelstan: "... honourably married his sister Eathgita [Eadgyth] to Sithric [Sihtric] ... who for love of the damsel renounced paganism and embraced the faith of Christ; but not long afterwards he repudiated the blessed virgin, and, abjuring Christianity, restored the worship of idols, and miserably ended his life shortly after his apostasy. The holy damsel thereupon, having preserved her virginity, abode at Pollesbury [Polesworth], persevering in good works unto the end of her life ..." This sister of Athelstan is said by some to be St.Edith of Tamworth; by others to be St.Edith of Polesworth.
Edward's second wife was Ælfflæd. William says she produced eight children. Two were boys: Ælfweard, who died just days after his father (in 924); and Edwin, who drowned, under possibly dubious circumstances, in 933. Of the six daughters, two (Eadflæd and Æthelhild) "vowing celibacy to God, renounced the pleasure of earthly nuptials". Eadgifu, Edward's third wife, had two sons: Edmund and Eadred, both of whom would succeed Athelstan as king; and two daughters: Eadburh, "a virgin dedicated to Christ"; and Eadgifu, "a lady of incomparable beauty", whom Athelstan married to Louis of Aquitaine. The remaining four daughters of Ælfflæd were also used to further English relations with the Continent. Between 917 and 919, Edward had given Eadgifu, to Charles 'the Simple', king of the West Franks.
In 911, Charles had concluded a treaty with the Viking leader Rollo. In return for Rollo's allegiance and adoption of Christianity, Charles ceded the territory which would become the nucleus of Normandy.
Charles faced a revolt in 922, and, in 923, was taken prisoner. (Apart from a short period in 927-8, he remained a captive until his death, in 929). Eadgifu brought their son, Louis, to live under Athelstan's protection in England. In 926, Hugh, duke of the Franks, sent a mission to Athelstan proposing a marriage alliance. Hugh sent Athelstan lavish gifts, including eminent Christian relics. Athelstan responded with equally rich presents, and, within the year, his sister, Eadhild, was married to Hugh. Similarly, in 928, the German king, Henry 'the Fowler', requested a sister of Athelstan for his son Otto. Athelstan sent two sisters - Otto chose Eadgyth (Edith), whilst Ælfgifu was passed on to, says William of Malmesbury, "a certain duke near the Alps" (thought to be Conrad of Burgundy).
William of Malmesbury writes that "... foreign princes with justice esteemed themselves happy if they could purchase his [i.e. Athelstan's] friendship either by affinity or by presents. Harold [Harald 'Fairhair'], king of Norway, sent him a ship with a golden beak and a purple sail, furnished within all round with a close-set row of gilded shields." Indeed, Norse tradition has it that Harald's son, Hakon, was raised in Athelstan's court - Egil's Saga: "Athelstan the Victorious, who was foster-father of Hacon the Good."
In 936, the throne of the West Franks became vacant. Declining the opportunity to take the throne himself, Hugh, duke of the Franks, proposed that Louis, son of Charles 'the Simple', should be recalled from exile in England to fill the vacancy. Following diplomatic exchanges with Athelstan, Louis was escorted across the Channel and crowned at Laon. Louis became known as Louis d'Outremer (i.e. 'from overseas').
Before he became king, Athelstan had stood as godfather to the son of Mathedoi, count of Poher - who had been forced to cross the Channel, following a Viking invasion of Brittany, in 919. Mathedoi's wife was a daughter of Alan 'the Great', last ruler of all Brittany. Their son became known as Alan 'Twisted Beard'. In 931, following his participation in a calamitous uprising against the Viking invaders, Alan was obliged to return to England. In 936, however, with Athelstan's assistance, Alan went back to Brittany, and soon afterwards established himself in his hereditary lands around Vannes and Nantes.
In 939, a dispute between Louis and Otto, son and successor of Henry 'the Fowler', over the control of Lotharingia, forced Athelstan to choose between his allies. Athelstan sent a fleet to assist Louis. The expedition was something of a fiasco - the fleet ravaged some areas of coast opposite England, and returned home without having taken any active part in the war. Nevertheless, as Sir Frank Stenton ('Anglo-Saxon England') notes: "... the war of 939, ineffective as the English part in it had been, deserves to be remembered as the first occasion on which an English king is known to have assembled a fleet in order to help a continental ally."
Despite this rift, the English royal family appear to have maintained contact with their German relatives - indeed, the ealdorman Æthelweard wrote his chronicle for Matilda, abbess of Essen, Otto and Edith's grand-daughter.