Well I finished the show and was extremely disappointed. In most Arthurian lore, Merlin isn't his servent- ever! He's an older guy (not specified how much older) who guides him to to the sword in the stone where he is the only teenager who can draw it out thus proving he is Uther's long-lost heir, Uther is long dead. Some say that sword is Excalibur and others say it's not. Arthur is supposed to bring about the new kingdom of Camelot and the future kingdom of Avalon. If this show stuck more to Arthurian legend, I would have liked it a lot more. I mean no love triangle between Arthur, Gwen and Lancelot? I think the tried with that dead Lancelot but it didn't work and he's a major character in the fable. Yet he dies so soon! Arthur should have known about Merlin's power from the get go, And I don't think magic was a common thing- I'll have to look in my notes (yes- how boring, I took a college class on Arthurian legend). Some say Marlin was a Druid himself.
I do believe it was an afterthought to have Morgana and Merlin to be actually related. Most lore says that they were raised like brother and sister but she turned on them and became a powerful sorceress.