The Magus Sisters are at Remiem Temple, which is southeast of the main area of the Calm Lands.

The exit from the Calm Lands to Remiem is accessible by chocobo only. There is a woman in the Calm Lands who will give a chocobo to you, but only after you find her and pass a chocobo training exercise. If you have not trained a chocobo, she will be in the northwest part of the Calm Lands. If you have trained to her, she will be close to wherever you enter the Calm Lands.

Once you are on a chocobo, go to the southeast corner of the Calm Lands. Ride up the slope towards the Macalania Forest exit, but instead of turning south to leave the area, continue east. There is a chocobo feather on the ground examine it to jump to the hidden exit. As with any Calm Lands exit, the game forces you to dismount before you leave the area. Now, you are at the outside of Remiem Temple. Simply walk across the bridge and enter the temple.

The interior of Remiem Temple holds just two things of interest: Summoner Belgemine in the center and the Chamber of the Fayth of the Magus Sisters on the right side. With Belgemine, you can fight versions of the aeons which you have obtained. The door to the Chamber of the Fayth is sealed; you cannot enter. To break the seal, you need the following:

1) Yojimbo (Aeon)
2) Anima (Aeon)
3) Blossom Crown (Key Item)
4) Flower Scepter (Key Item)

Flower Scepter - Obtain it at Remiem Temple. Defeat Belgemine's Bahamut, and she will give it to you as a prize.

Once you have both aeons and both Key Items, you can open the door in Remiem. Go inside and the Magus Sisters are yours!