Gwenddydd
Sister of Myrddin (Welsh prototype of Merlin) in early Welsh lore.
Her son apparently fought at the battle of Arfderydd, where he was killed by Myrddin, after which Gwenddydd disavowed her brother. Myrddin was driven insane by the killing. Several poems include conversations between Gwenddydd and Myrddin.
Geoffrey of Monmouth calls her Ganieda, appearing in the Vita Merlini, where she appears as the philandering wife of Rhydderch Hael, whose infidelities were spotted and reported to her husband by her twin brother, Myrddin.
Source
Various; all Myrddin poems | 12th century to 15th century