In the Vita Merlini she is the wife of King Rhydderch of Cumbria and her adultery is spotted by Merlin. She despaired when Merlin went insane, and sent out knights to find him. When Merlin was brought to court, he told Rhydderch of Ganieda’s adultery, but Ganieda managed to convince her husband that nothing Merlin said could be trusted because of his madness. After her husband died and Merlin was restored to her wits, she went to live with her brother in the forest of Caledon. There, she developed, like Merlin, the ability to prophecy.
The idea of her being an adulteress may have stemmed from Jocelyn's Life of St Kentigern in which Rhydderch's wife, Languoreth, becomes enamoured of a soldier. The Welsh poems do not say definitely that Ganieda was married to Rhydderch.
Glennie does not make it clear whether she, also, was a necromancer. In Namer of Beasts, Maker of Souls, an exquisite 1995 Arthurian novelette based on a kabbalistic interpretation, Jessica Amanda Salmonson refers to Merlin's twin sister Ganicenda as Divine Wisdom, with her head in heaven and her feet in Sheol.