Is the "Sithis" the Dark Brotherhood worship just a proxy by Mephala, the patron of the Morag Tong? I know the Night Mother and the brotherhood were directly associated with Mephala in Daggerfall, and my general understanding of the concept of Sithis leads me to think the Dark Brotherhood are full of [NUMINIT].

You are most likely correct, but you won't find any explicit confirmation in any texts.

Nobody really thinks that Mephala is Sithis, not even the Sithis of the Dark Brotherhood, but it's a common theory that Mephala is the Night Mother. If she truly is, it reveals and interesting divisive side to her. Pitting her champions against each other. And if that is one of her attributes, it certainly explains why Mephala, like Boethiah, didn't step in during Azura's feud against the Tribunal; if Mephala is divisive and enjoys seeing her champions fight one another, she'd enjoy such a spectacle as the Tribunal. I'm sure that Boethiah enjoyed it.