User blog comment:Merlin919/Is Morgause, Vivienne and Gorlois's daughter/@comment-5352424-20120820104504

Originally, Gaius said that Morgause was Morgana's half-sister, when he didn't know that Uther was Morgana's father. Later, when Morgana told Morgause this, she still adressed her as sister afterwards. This scenario implies that Morgause's father was not Gorlois anyway.