Here’s a little family tree primer:
Anna Pendragon is our heroine and narrator. She’s the sister of Arthur. Gweyn is Arthur’s wife.
Together with her first husband, Lot, Anna had Gawain (also Gaheris and Gareth, but they come later). While Arthur is her full brother, she also has three half-sisters: Elaine (not pictured), Morgen, and Margawse. Bedevere is Arthur’s right-hand man, and also Anna’s old paramour. Vyvian is her aunt. Merlin and Lanceloch, well, they aren’t directly related, and too much about them might make some spoilers.