Has anyone else suggested Carla's excellent Paths of Exile- it's about the young Edwin of Northumbria in the years before he became king.Hence the request for more suggestions pre-King Alfred time period.
Also recommended (but unfortunately OP), Kathleen Herbert's Cumbria trilogy, which all feature members of different royal families and are set in the 6th/7th centuries:
1) Bride of the Spear (aka Lady of the Fountain)
Follows the difficult relationship between Prince Owain, son of King Urien of Cumbria and Taniu, daughter of King Loth of the Gododdin
2) Queen of the Lightning
Riemmelth, the "Queen of the Lightning" of the title, is the last of Cumbria'a royal line. To ensure the safety of Cumbria, Riemmelth is forced into marriage with her hated enemy, Oswy, a Prince of Bernicia.
3) Ghost in the Sunlight
Riemmelth's daughter, Alchflaed of Bernicia, stirs up dark secrets from the past when she acts as peaceweaver and marries Peada, the son of Penda, king of Mercia.