I’ve seen mention of them in a few of the EU novels (I’ve only read one or two of the TPB’s, no comic books, so I’ve got a lot of holes in my EU knowledge). From what I gather, they’re an all-female order of Dark Side of the Force users. That’s about all I know.
Can anyone tell me which, if any, of the novels they figure in as, like, actual characters? Any TBP’s I should be trying to hunt up?
Please feel free to spoil, and to omit any- and everything written by Darth Anderson.
They played a major role in The Courtship of Princess Leia (which sounds from the title like it would be absolutely awful but wasn’t that bad).
Nailed it in one.
It’s also considered by some to be the home planet of the Rancors (“some” including the ushers who were having the Star Wars trivia contest to determine who got to be seated first for Revenge of the Sith. I won).
I’m not sure if they’re necessarily “Dark Side”, or if that’s just Jedi propaganda against non-Jedi force users. 
But yes, they do have force powers, have a matriarchal society and they do keep rancors as pets/mounts. A few EU sources have mentioned them in passing (usually when Luke is looking for more students for his Jedi Academy), but I haven’t read any of the ones where they were a major focus.
Dathomir is a planet included in the “Star Wars Galaxies” MMORPG, and the Night Sisters are a pretty nasty force in that game, along with their enemies the “Singing Mountain Clan”. I’m not sure if that Clan is a Night Sister offshoot, or a separate group of locals fighting over the same territory. My SWG character isn’t quite up to the challenge of Dathomir, so I’m not sure whether the Singing Mountain’s are force users or not.
The Singing Mountain Clan is the original group of Force-users on Dathomir. The Nightsisters found that using the Dark Side was more effective and split off from the Clan to form their own little group (sound familiar? ;)). The Singing Mountain Clan literally sings spells to use the Force, which functions as a sort of mantra-like focuser for their powers.