The Magus Sisters are an optional Aeon that you can get in the game, who are quite powerful. In order to get them, you need to do two things. The first is to collect one of every fiend on Mount Gagazet, which will get you the Blossom Crown. The second thing is to go to Remiem Temple and fight Belgemine’s Aeons, until you get to and defeat her Bahamut. You will be rewarded with the Flower Scepter for doing this. Once you have both items, head to the sealed door behind Belgemine and use them to unlock the seal, letting you recruit the Magus Sisters.