What was king arthur's sisters name?

Morgan LeFay

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He actually had three half-sisters according to malory in Le Morte d'Arthur. The three daughters of Igraine and Gorlois, the Duke of Cornwall were: Morgause, Elaine and Morgan le Fey.

According to Geoffrey of Monmouth in the History of the Kings of Britain Arthur had a full sister named Anna.

It appears that Morgan le Fey maybe called Fata Morgana