I'm not sure if it's specifically said anywhere in the series, but is her last name really Mikaelson? If one assumes that the original family are of viking decent (which names, time line and circumstances point toward), the convention of the time is for the daughters to get their mother's name as a base for their last name with the addition of the suffix -daughter. So her name should really be Estherdaughter or at the very least Mikaeldaughter.
As I said, I am not sure where in the series, if ever, the originals are refered to as having the last name Mikaelson. If it is simply a convention outside the VD universe just as to give them a family name or if it's used in purposes where a last name is needed in this day and age (identification and such) I guess it's understandable, albeit not fully correct.
Anyone else have any thoughts on the matter?