sisters

I watched Merlin when it came on originally and have forgotten most of it. I have just restarted the series today. So, I can't say for sure in the series. But, I just read The Mists of Avalon and in that telling of the Arthurian legend, they were not sisters. In The Mists of Avalon, Igraine was the mother of Morgaine, aka Morgana (by her first husband, the Duke of Cornwall) and Arthur (By Uther, of course). Morgause was Igraine's sister, and therefore Morgaine's aunt. Igraine and Morgause had a third sister, Vivian, who was the Lady of the Lake and high priestess of Avalon. In The Mists of Avalon, the women had fairy blood, and that is where they got their powers from. It's far more complicated than that. But, I kept it simple for the purpose here. If you like the Arthurian legend, I highly recommend reading The Mists of Avalon. I listened to the audio book, and it was great.

I've been watching and am almost done with the series now. They have the same mother, Vivienne. She was the wife of Gorlois, the man that Morgana thought was her father before finding out she was Uther's daughter.