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.