The main character I play is super devoted to the Dark Brotherhood (lots of backstory dumping for the full reason why), and so naturally she worships Sithis and the Night Mother. But in the game, you can't really do that, not the way you can worship the divines and get healed of diseases and perks from them. I've looked at a few religion mods, but they all seem to add a lot, and I was looking for something a lot more lightweight. A shrine in each of the sanctuaries (and if there's compatibility for The Brotherhood of Old, even more awesome, but beggars can't be choosers), and maybe just a small bonus to something like sneaking or sneak attacks? Nothing bigger than what the original blessings offer.
If I knew literally anything about modding, I'd try to make it myself, but my knowledge of modding is that I have absolutely no clue what anyone is talking about when they try to explain how to do it. If a mod like this does already exist and I've just missed it this whole time, I'd love to hear about it, but as I said, the only religion mods I've encountered that add Sithis worship add a bit too much for me. (And as much as I'd love to learn how to mod one day, this year really is not the year for it. Things have been rough, and I don't think I could handle the frustrations that come with the inherent failures of learning.)