In Morrowind, Severa Magia, a target of the Morag Tong, is identified as the Night Mother of the Dark Brotherhood and is killed. In Oblivion, the Night Mother is a spirit. You figure out the rest.
Any comments?
http://www.gamesas.com/bgsforums/index.php?showtopic=1071229&st=0
Other than that, here's a simple answer: The title of Night Mother takes many forms at once. The first that come to mind are: Mephala acting under the guise of the "Night Mother", on Nirn as an avatar (such as the Oblivion ghost and what powers the altars) or a mortal assassin taking the title of Night Mother as a sign of rank (Severa Magia and the old woman from the Thieve's guild). There are other instances, but these are the two most commonly brought up. That other thread goes into more detail, and I can later, if you still want to use this thread.
EDIT: Oh, and if you are trying to get at Severa being the ghost in Oblivion, please note that the ghost is a Dunmer while Severa was an Imperial.