Anna

According to Geoffrey and his successors (Wace and Layamon), Anna was the daughter of Uther Pendragon and Igraine. Which meant that Anna was the full sister of Arthur. Anna was probably one and the same person as Morgawse, the half-sister of Arthur, in the later legend (see Morgawse).

But her parentage and her relationship with Arthur was not so clear in Geoffrey of Monmouth's account of Historia regum Britanniae. There was even confusion on who Anna was married to.

At first, Anna was the daughter of Uther and Igraine, and therefore the full sister of Arthur [VIII 20]. And in the next paragraph [VIII 21], Uther gave Anna to Lot (Loth) of Lodonesia in marriage. But earlier on, when Arthur was ruling the kingdom, Hoel, king of Brittany, was said to be the son of Anna (Arthur's sister) and Budicius, king of Brittany [IX 2]. So here, we have a contradiction as to who Anna's husband was.

Later still, when Geoffrey said that she was Lot's wife, and the mother of Gawain and Mordred; Anna was no longer the sister of Arthur and daughter of Uther. No, she was now the sister of Aurelius Ambrosius, which made her as Arthur's aunt [IX 9].

It's a possibly that there were two Annas, one who was Arthur's sister while the other was Arthur's aunt. Another possibility, in the case of there being two sisters of Arthur, both are named Anna - one married to Loth and the other to Budicius of Brittany. Both are possibilities, but it is highly unlikely. I would just say that Geoffrey of Monmouth made a blunder when he drew up Arthur's genealogy. There was no such confusion found in the accounts of Wace and Layamon; Anna was clearly Uther's daughter and Arthur's sister.

Although her name appeared several other times in other works (eg The Rise of Gawain), her name was replaced with Morgawse or various other spellings. In many of these later legends, Morgawse was only Arthur's sister; see Morgawse for more detail.

In the Welsh legend, the sister of Arthur and mother of Gwalchmei (Gawain) was named Gwyar. Though none of the tales of the Mabinogion say who Medrawd's (Mordred's) mother was, we can assume his mother was also Gwyar. It is believed that Gwyar was thought to be a goddess or the Sovereignty of Britain.

And according to Wolfram von Eschenbach, a German poet who wrote Parzival (c. 1200), the sister of Arthur was named Sangive. Her children were named Gawan (Gawain), Beacurs, Kundrie, Surdamur and Itonje.