Do you really need a name for them to be memorable?
I'd think the Coven would have a one or more simple names for themselves that described precisely what they are... The Servants of Wee-Jas, for example. When together and referring to each other, it'd be easiest and proper for them just to call each other "sisters".
The local townspeople however, would be superstitious about them, and the Sisters would, of course, be tight-lipped toward them. I can imagine the locals speaking about the coven only in hushed whispers, so as not to draw their attention. If pressed, they'd refer to them simply as "The Witches" with an accompanying hand gesture to ward off the Evil Eye.
But suit yourself, it's your game.