He was the youngest of six children. His eldest brother, Æthelstan, was old enough to be appointed sub-king of Kent in 839, almost 10 years before Alfred was born. He died in the early 850s. Alfred's next three brothers were successively kings of Wessex. Æthelbald (858-860) and Æthelberht (860-865) were also much older than Alfred, but Æthelred (865-871) was only a year or two older. Alfred's only known sister, Æthelswith, married Burgred, king of the midland kingdom of Mercia in 853. Most historians think that Osburh was the mother of all Æthelwulf's children, but some suggest that the older ones were born to an unrecorded first wife. Osburh was descended from the rulers of the Isle of Wight. She was described by Alfred's biographer Asser as "a most religious woman, noble by temperament and noble by birth". She died by 856 when Æthelwulf married Judith, daughter of Charles the Bald, king of West Francia.