In Geoffrey of Monmouth's History of the Kings of Britain, we find confusion concerning the relation of Hoel to Arthur. To outline the issues as presented:

1) Uther and Ygerna have a son and a daughter: Arthur and Anna.

2) Anna is given to Loth by Uther as a reward for his bravery. They have two children: Gawain and Mordred.

3) Hoel is the son of Arthur's sister (unnamed by Geoffrey) whose father is Budicius

4) Geoffrey says at one point, "Loth, who in the days of Aurelius Ambrosius had married that King's own sister..." which could indicate either Arthur's sister or Aurelius' sister.

5) Geoffrey repeatedly refers to Hoel as Arthur's cousin.

   Now this makes Hoel either Anna's son by a different marriage (to Budicius before Loth, as Loth appears later in the narrative), the son of another of Arthur's sisters who is not mentioned by name, or strangely the son of Arthur's aunt, Anna (Aurelius' sister). In the Penguin Classics edition translation by Lewis Thorpe, the note on p. 214 indicates that Hoel should be seen as the son of Aurelius' sister making him Arthur's cousin, and the mistake is Geoffrey's.