Not too long ago I was talking to someone about this. I was thinking that Mab is Morgana or Morgause, and one of her sisters is Titania.
The Athame is supposed to be on par with the greatest of the Swords, but Jim said it wasn't so much what the Sword was, than who it belonged to. Just being Morgana's and being used in a ton of high-level magic might be enough, but that's some *serious* juice to have for just being passively charged, the Swords were made by a Cosmic event, and this athame was made just from just one Wizard lady, even if a crazy-powerful one? One would think that there'd be at least a few more things like that floating around if it was that simple. (Of course this is all hindered a bit by the presence of Nemesis, which might have made an unequal trade possible.)
If Mab was Morgana though, then it all makes sense. Lea didn't want it just because it was powerful, she wanted it because it was Mab's or Titania's. It would mean that the blade has a strong personal connection to a current Queen, and *that* warrants it having serious power, and would explain why Lea having it caused such a big power imbalance that Mab felt she couldn't be allowed to keep it.
That would put Mab as a contemporary (and possible apprentice/lover/foe) of the original Merlin (Hmm, Molly/Harry parallel? See: Mary Stewart), a sister to a Knight of the Cross, and she'd have been a legendary badass even before becoming Lady. The timeline fits since we know by WoJ that Nicodemus is older than Mab.
Around Arthurian Times Mab becomes Lady, and probably around the Battle of Hastings becomes Queen.
This is where it gets muddy though. In *The Once and Future King* series, Morgana and Morgause get merged. It would seem that Jim drew from this at least a little, since one of the books is titled "The Queen of Air and Darkness".
There's lots of versions of Arthurian legend, so it depends a lot on where Jim chooses to draw most from, and it's likely that he'll pick and choose from various places as-needed, and put his own spin on it all.
Until proven otherwise, I'm solidly in the belief that Mab and Titania are Morgause and Morgana (Elaine, the third sister wasn't really a primary character). I was more thinking that Mab was Morgana until I read about the Once and Future King Series, after that, it seemed obvious (almost tooobvious though...).