No, no, no. When someone asks a lore question, we don't link them to wikis, we move the discussion to the lore forum. You'll get answers soon.
Is she actually Mephala incarnate?
There are a good number of scholars who believe this. It makes a bit more sense for the Webspinner to be playing around with the lives of mortals than Sithis, which is change.
What is Mephala's relationship to Sithis?
Sithis is the Primal Force Padhome. Mephala is a Daedra. Daedra are generally Padhome-aligned et'ada.
Why are the Dark Brotherhood and the Morag Tong rivals?
http://www.imperial-library.info/content/brothers-darkness
Is Sithis an Aedra gone bad?
Nope.
Why is Mephala considered a "good" daedra?
She is really only considered "good" by the adherents of the Tribunal Temple, because she is the http://www.imperial-library.info/content/anticipations of Vivec
What power does Sithis or the Night Mother truly have other than wispering to mortals?
Again, this depends on your interpretation of the Night Mother's identity. If she's http://www.imperial-library.info/content/mephala, then she's got lots.