Obtaining Magus Sisters

You haven't yet obtained Anima and Yojimbo, have you? You need both of them to be able to obtain the Magus Sisters.

Some of the Earlier versions of the Hidden Aeon FAQs never did mention about having to obtain the 2 other optional Aeons, Anima and Yojimbo as a prerequisite to getting the Sisters.

Anyway, to get Anima, you need to get all the Destruction Sphere treasures and head to Baaj Temple (I forgot the coordinates) and then face Geo!, the rib-cage-exposed fish. You then just proceed to the temple (marked by the green marker) and Anima is yours.

For Yojimbo, head to the ravine below the bridge connecting the Calm Lands to Gagazet and head inside the Cave. Move forward and you'll have to fight Lady Ginnem (assuming you haven't visited earlier like your first visit to the Calm Lands) and after that, head to the Chamber and have a bidding war with Yoji!

You don't need the Celestial Mirror to obtain Anima, Yojimbo, or the Magus Sisters, but you might as well get it, considering you'll need it to obtain any and all Celestial Weapons.