With no explanation, label text_A→text_B with either "DON'T KNOW", "NO" or "YES".
text_A: Oswald seems to have been on good terms with the West Saxons: he stood as sponsor to the baptism of their king, Cynegils, and married Cynegils' daughter. Her name is reported by only one source, Reginald of Durham's 12th century "Vita S. Oswaldi", which says that it was Kyneburga. Although Oswald had one known son from his marriage to Cynegils' daughter, it is uncertain if Æthelwald was this son, or a second son from an earlier relationship—since Æthelwald began ruling in Deira in 651, it has been argued that a son from the marriage to Cynegils' daughter would have been too young at the time to be trusted with this position, and therefore may have been older, the product of a relationship Oswald had during his exile.
text_B: Would a historian researching the life of Oswald feel there was a good chance of discovering evidence of a brother to Æthelwald?
YES.