http://www.uesp.net/w/images/Shrine_of_Sithis.jpg
Obviously, assumingly, Sithis has not been seen. Regardless, the odd statue interests me.
According to UESP, of whom I don't whole-heartedly trust, but of which, for the sake of time, I will use: (In my own outline, but in their words.)
Some scholars also argue that when the Morag Tong was banished from the rest of Tamriel, they were allowed to continue to operate in Morrowind when they replaced their worship of Mephala with that of Vivec. As a reaction to this the Dark Brotherhood was formed, still worshiping and being led by the mysterious Night Mother, better known to Tamriel as the Daedric god Mephala
Daedric Princes have incredible power, and can take on many forms. So, for one to be the Night Mother, is not out of the question. Especially since Mephala is seen as loving executions (Morag Tong and DB) and lies (obvious). But I cannot see her creating something so elaborate, and them worshipping "someone else" and killing in someone else's name. If I am not mistaken, Mephala, being who she is, would be selfish in taking credit for what she does.
As per Sithis in specific -
He is referred to by some texts and legends as a child of Mephala, while some say he is Mephala (just in different form and appearance)
Again, I cannot see Mephala letting someone else take the credit, granted it may be her, it's towards another, or false, god. Being a child of Mephala can make more sense. But, why, if he's a child of Mephala, would he be considered to pre-date all?
If Mephala has a child, I would assume, imo, he'd carry on some of the traits of his mother. Specifically - Lying and executions. And, in this, I believe it makes more sense for him to take on the role of Sithis. And, perhaps egotistically, he wants to be seen as all powerful.
Now, yes, this is all speculation. And it is not a complete, formed thought. This is on-the-fly thinking and ideas as I typed. I hadn't really thought about the whole DB/Sithis/etc thing before. I just took it as it was and considered the Sithis to be their name for Chaos, while others call him by a different name. (Basically, they gave a new/different persona to something, just like every other race/clan/village/faction/etc does.)