I like to work with the idea that Sithis is Mephala trying to confuse its followers. we already know Mephala has a tendency to take different forms but also you ahve to give a reaol life context to this. somewhere out there there is a god of Gravity. i've also speculated that Mephala is a being closer to Sithis created as a perconefication or mediator to communicate with Muundus
Indeed, the Sithis that the DB try to worship, the one who makes blessings appear at their shrines, hears the Night Mother prayers, gives the Listener the names, and powers the ghost lady under the statue is all speculated to be Mephala messing with her followers. Unfortunately, the metaphor only goes so far, Sithis is not actually responsible for gravity. That is probably Lorkhan/Shor, Sithis is the primordial state of change, the void, Is-Not, Padomay, etc. The opposing force being stasis, or Anu, which together created the gray maybe of the Aurbis, in which all things are (the Aetherius, Oblivion, the Aedric planes, etc.)
While Lorkhan and Mephala might have close ties with Sithis, probably being two of the most Padomaic gods out there, they are not the actual Padomay. I'm not totally clear on the difference between Sithis and Padomay, it might just be two names for the same thing, but I'll need a second opinion on that one. Thus those two are responsible for the biggest forms of change, Lorkhan changed everything by creating Mundus, and Mephala's realm is all about stirring up change. From Vivec and Mephala:
As known in the West, Mephala is the demon of murder, six, and secrets. All of these themes contain subtle aspects and violent ones (assassination/genocide, courtship/orgy, tact/poetic truths); Mephala is understood paradoxically to contain and integrate these contradictory themes.
Vivec (who is the anticipation of Mephala) was a major point of change, disrupting the stasis once again, showing Mephala's padomaic nature. The Morag Tong did the same thing when they killed the Potentate, and then they themselves erupted into change and chaos when the Brotherhood branched off. So basically all of Mephala's weird motivation is based on a natural urge to disrupt the complacent order of things. Lorkhan is much the same, his avatar (Pelinal) changed the world from the Ayleid city states to the Imperial Empire we know today.
So, while they share Padomaic traits, this does not mean that they are direct manifestations or followers of Sithis. But in the end a lack of canon information causes the trail to stop cold after all the racial bias. Perhaps the Hist know?
I thought that the Murdering side of the DB was led by the nightmother, or Maphalia, and their religion is a messed up worship of Shor (Aedra are what you think they are)
I'm not entirely sure where you got Shor from, as the only time I see him with the Dark Brotherhood is in the statue. And though we cannot discount its influence, Grewyn is by no means a viable source of canon information, being a crazy vampire assassin who was killed by the Dark Brotherhood. He might have thought that Sithis/Mephala took the form of a hole-in-his-heart mummy, and in his visions Mephala very well might have shown herself as just that, but that doesn't give us a base to make reliable assumptions. I admit Shor is tied in there with Mephala, as they are both very Padomaic, but I can't find anywhere linking the two conclusively, especially since Mephala didn't participate in creation with him.