The lunar Magus Sisters are in the artwork for the Thavnair tower dungeon, so I'm assuming we're going to fight their knock-off versions there then face the real deals (As a real as a primal can get, at least) a fair bit later after they've had some time to fade from memory.

There's another outdoor Thavnair dungeon that seems to take place after the world starts going to heck from the Final Days, so I'm thinking they'll probably decide to show up around then.