Final answer:
In Morte d'Arthur, Arthur and Mordred are cousins.
Step-by-step explanation:
In Morte d'Arthur, Arthur and Mordred are cousins. They are both descendants of King Uther Pendragon, but through different bloodlines. Mordred's mother was Arthur's half-sister, while Arthur's father was Uther and his mother was Igraine. This familial connection adds complexity to their relationship and contributes to the overall tragedy of the story.