When I examine the morag tong they seem to be more business like as time went on while the brotherhood is more of a cult than anything else. I mean I could imagine that mephala would get upset that the tong now worshipped vivec to continue their existence and the dark brotherhood did not like this so this is what caused the schism to happen. Well this is one of the theories one that I believe actually.
I mean according to this book
http://www.imperial-library.info/content/fire-and-darkness
Mephala is the night mother or could be an aspect of her. The more I look into this the more I start to believe that the dark brotherhood are actually worshiping Mephala. I mean the way the night mother talks to you in skyrim and when you do the whispering door quest to me it just feels similar. Well books in elder scrolls are not really conclusive evidence but there other things in the lore that kinda points to it being her or an aspect of her.
Anyway I can be could be completely wrong but there are some links that point out that may actually be her. What do you lot think? People here know a lot more about lore than me so I am quite interested to know your thoughts on the matter well anyway the more I look into it the more true it seems to be, well for me that is.