larking around today and saw this... so...
Nightsisters are witches from all clans that used Alyan magic (the force) with dark intent. As a result of that mistake they are casted out of the clan and sent to survive on their own. Over time the outcasts formed a clan and with their anger/ hate towards those that banished them, they worked/ moved to take over their world and wipe all light side clans off the face of Dathomir.
Now in the SW: TCW series, the Nightsisters were using their Nightbrothers to gain an upper hand against the other clans. Use the Sith to get people off world and gain resources to build their own power. People make deals with other people... One dark group made a deal with another dark group. That is all. To Nightsisters and their pride of who they are, it is insulting to call them Sith since they see Sith as Dark Jedi and Jedi are just another group they hate cause of their interactions with Dathomir.