Sithis is the embodiment and personification of Chaos and Change, opposed to the Order and Stasis of Anuiel, and there's no direct connection to necromancy. The Dark Brotherhood accepting Vampires is more so about them not really caring about who you are as long as you can kill and adhere to the Tenets. The Wrath of Sithis manifests as a Wraith, but I don't think it's any kind of traditional undead, so no real connection to Necromancy there either.

As for the Dark Guardians protecting Sanctuaries, we don't really know much. Perhaps they are created through Necromancy, or summoned from the Void.

Personally, for a character like that, I'd pick Nightblade as the base class to get access to both Assassination and Shadow to lean more into the Sithis aspect, then pick up one of the Necromancer skill lines, but obviously it's your character and I don't know their story, so you do you