My entry for the "Arthurian Legends" contest being run by profilic-imp: [link]

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In the tale told by Geoffrey of Monmouth, titled Vita Merlini (c. 1152), Ganieda was known as the sister of Merlin. In the Welsh legends, Ganieda was known as Gwenddydd, and was the sister of Myrddin (Merlin). She was also the sister-in-law of Gwendoloena, Merlin's wife.

Ganieda (Gwenddydd) was the wife of King Rodarch of Cumbria (Welsh Rhydderch Hael, king of Stathclyde).

According to Geoffrey of Monmouth, when her husband died, Ganieda lived in the forest with her brother. It was she who had built a house with seventy doors and seventy windows, for Merlin. The construction of the house allowed her brother to observe the night sky and foretell the future (astrology).

The legendary bard Taliesin later joined the brother and sister in the forest, where the bard shared news with Merlin. When Merlin regained his sanity by drinking water from healing spring, Ganieda gained the power of prophecy, supposedly superior to his own power.

As the Welsh Gwenddydd, the poems attributed to Myrddin (Merlin), she was upset with her brother for the death of her son. But later she was reconciled with Myrddin, in a dialogue between Myrddin and Gwenddydd (Cyfoesi Myrddin ac ei Chwaer Gwenddydd). Here, we find out that their father was named Morfryn, and that he and her sister were twins.

Ganieda doesn't appear in the mainstream Arthurian legend. However, some people see that Merlin's sister was replaced with a fairy woman and sorceress known as the Lady of the Lake. The Lady of the Lake is known by several different names, including Niniane, Nymenche or Uiuiane (in the Vulgate Cycle), Nimue (in Sir Thomas Malory's Le Morte d'Arthur) and Vivian (a name more popular in modern literature).

That's not to say Ganieda is a Lady of the Lake; she may be the antecedent of the Lady of the Lake. Though, Merlin's sister and the Lady of the Lake appeared to be two totally different people, the resemblances are there. Ganieda became gifted in divination as soon as her brother's sanity was cured. Therefore, Ganieda had inherited Merlin's position, since her brother refused to prophesy any more. Merlin said that his sister was even better than him in divinition, just as the later Merlin was supersceded by the Lady of the Lake. While Niniane (Vivian) was a pupil in Merlin, she learned all she could about magic from Merlin. After confining or killing Merlin, Niniane inherited Merlin's magical knowledge and skill, and became Merlin's successor as King Arthur's adviser.