There's always alot of stuff talked about Sithis, but after looking at the monomyth it reads like Anuiel is like the other half of Sithis, so why don't we ever hear about it. What is Anuiel's significance? Surely its something with how important Sithis seems to be.

Sithis gets talked about allot recently because the Dark Brotherhood is treating him as a god, somewhat odd since Sithis half of excistence, Anuiel being the other half. With excistence essentially being their interaction, it's hard to ascribe any sort of event to Sithis.

It's a bit odd, but looking at the details there is enough to ensure that Sithis isn't the next Daedric Prince, the Darkbrotherhoods concept of a Dread Lord can find purchase in Lorkhan and has it's origins in what used to be a philosophy rather then a cult-religion in the Morag Tong.

In the lore itself Sithis is not that significant.

You hear about Anuiel plenty. The elven Auri-el (Akatosh) is a direct descendent. You also may know him as Anu, whose name changes meanings depending on what it is couple with in a sentence.

Exactly. You hear about Sithis, but not http://imperial-library.info/mwbooks/sithis.shtml

From what I see, the Cyrodiil Dark Brotherhood's myths shape the Sithis that they interact with, like with just about any non-Daedric god, so they get what they believe. In contrast, the Morag Tong are ultimately instruments of Mephala, a Daedra.

Where Sithis is essentially the Void (nothingness, Chaos, blah blah blah), Anuiel is the Totality of Creation, the thing that fills the void of Sithis and allows existence.

But if the universe is made up of the interplay of Sithis and Anuiel, how can Anuiel be the "Totality of Creation"?

Because Sithis is nothingness. Existence stems from Creation and Nothingness going at it like two drunks knife-fighters on a Tuesday night behind Jimmy's. An easier description of Anuiel is the Soul of Anu. And Auriel is the Soul of Anuiel.

Yes, though what they got doesn't come fresh of the god-factory as a single never used before product shrink-wrapped and all. Like everybody else they end up some second-hand idea to fill in for it, it might be dressed in funny robes, black make-up and speak the cool tongue, it's still going to be tied that idea.
It's those ideas that spontaneously took shape in the first place because they -even without people- resembled some fundamental concepts like love, law or knowledge. Since it's those concepts that garner the power for action, you have to shape your myths to match them so you too can leverage their power.

Hence the statue of Sithis looks like Lorkhan. He's the guy that brought death back to the Auribs and together with Mephala, the mother of secrets, they gave birth to a cult dedicated to murdering people in secret. It's their combined concepts that make the Morag Tong and the Dark Brotherhood possible.

Works better if you describe one as everything there is, was, will and could be, the other as the space to act out one of those events. Like how you can't build a sand castle in a room full of sand, you need space for that.