>>12531348>Gawain's sister Guineveire.
stop right there. Don't make up your own Arhturian mythos.
Guinevere is Leodegrance's daughter, Gawain, like Gareth, Gerheris and Agravain, is one of Lot's sons, and one of Arthur's nephews. They were all half-brothers to Mordred of course.
Guinevere is related to none of them, except by marriage to Arthur.