What do we know about Guenivere's sister Guenivak (Gwenhyvach)? I have only read about her in Bernard Cornwell's books (when she's ugly and hates Guinevere), and I read on the Internet that she and Mordred were responsible (directly or indirectly) of the battle of Camlann.
Yes, she basically appears only in some Welsh Triads and probably in Lancelot-Grail (in Lancelot-Grail there is a false Guinevere who manages to take Guinevere’s place and she is usually considered Guinevak).
The Welsh Triads are these ones:
TRIAD 53Three Harmful Blows of the Island of Britain:The first of them Matholwch the Irishman struck upon Branwen daughter of Llyr;The second Gwenhwyfach struck upon Gwenhwyfar: and for that cause there took place afterwards the Action of the Battle of Camlan;And the third Golydan the Poet struck upon Cadwaladr the Blessed.
TRIAD 54Three Unrestrained Ravagings of the Island of Britain:The first of them (occurred) when Medrawd came to Arthur’s Court at Celliwig in Cornwall; he left neither food nor drink in the court that he did not consume. And he dragged Gwenhwyfar from her royal chair, and then he struck a blow upon her;The second Unrestrained Ravaging (occurred) when Arthur came to Medrawd’s court. He left neither food nor drink in the court;
TRIAD 84Three Futile Battles of the Island of Britain:One of them was the Battle of Goddeu: it was brought about by the cause of the bitch, together with the roebuck and the plover;The second was the Action of Ar(f)derydd, which was brought by the cause of the lark’s nest;And the third was the worst: that was Camlan, which was brought about because of a quarrel between Gwenhwyfar and Gwennhwy(f)ach.This is why those (Battles) were called Futile: because they were brought about by such a barren cause as that.
