As a child Athelstan was a member of a large and complex family. He was the first child of King Edward the Elder. We know little about his mother but later tradition names her as Ecgwynn. Edward and Ecgwynn also had a daughter who was perhaps called Edith. William of Malmesbury questioned whether they were properly married and suggested that Athelstan was illegitimate.

At about the time that Edward succeeded to the throne in 899 he married another woman called Aelfflaed. We do not know whether Ecgwynn had died or was simply replaced as Edward’s partner. Edward and Aelfflaed had several children: two boys and six girls. Aelfflaed died in around 919 and Edward remarried. His third partner, Eadgifu, had at least three children 919-924: two boys and a girl. So, in total Athelstan had at least twelve siblings: one full sister, seven half-sisters and four half-brothers.