We don't even know that the final Cylon knows they are a Cylon. Who knows if they've even been shown in the series yet. And it can't be Ellen Tigh, because she is dead, and the Final Five don't just pop back up on a Basestar like the other Cylons. I think, flat out, they are not *of* the other Cylons, they are entirely something different (read: older).
And, again, its been said a million times, Kara is not a Cylon. Could they be BSing us? Sure. Moore said for a time that he wasn't going to off Pegasus either, just because everyone was expecting him too. But right now I'm going with the official line that she's not.