I'm a bit of a newbie to TES, but unless I read it wrong, the book 'Sithis' states that Sithis created Lorkhan, and that Lorkhan was meant to infiltrate the Aedric pantheon and in turn seed the world with the cliche 'Pandora's Box' variety of woes, fears, and ills.
Sithis is half of the pair of forces who's interplay created the world. As such Sithis doesn't act because all that we can perceive as Sithis actions are in interplay with Anu. It's only for convenience that Sithis gets personified.
Another thing to keep in mind is that just because it's written down somewhere, it doesn't make it the absolute truth. It's only a view point from somebody, in the case of "Sithis" that would probebly be the Dunmer priest, guessing by the reference to Vivecs Sermons.
Anyway
http://www.imperial-library.info/mwbooks/monomyth.shtml.