I am confused, in the beginning - Arthur mentions that he and Morgana come from the same womb... but that wouldn't be true. He is not a Le Fey, he is Uther's son but not the son of the woman his father SA'ed. If he were, they'd be full siblings and know about each other. Likely, Uther fathered Arthur with a commoner or an initial wife and then stormed in an assaulted the noblewoman he forced into marriage and had Morgana with her.
Viewing post in The Bastard of Camelot comments
Arthur and Morgana have the same mother - Igraine - and different fathers - Uther being Arthur's, Gorlois being Morgana's. It's Igraine who is the Le Fay, so the one whose bloodline carries magic; but just having magical blood doesn't ensure one actually has active powers and is thus a sorcerer. So Igraine, Morgana and Arthur are all Le Fay but only Morgana has the magical powers.
Also, while Morgana is not Uther's bio kid, she was legally adopted by him - it's why she thought she had a claim to the throne.