To get the Magus Sisters, capture all of the required types of fiends on Mt. Gagazet and return to the monster arena owner in the Calm Lands. As a reward, he gives you the Blossom Crown. You must also possess all of the other aeons, including Yojimbo and Anima, which are hidden aeons in the game. Next, go to Remiem Temple in the Calm Lands.

After you defeat Spathi, Belgemine will give you the Flower Scepter. After defeating all of the aeons at Remiem Temple, use the Blossom Crown and Flower Scepter on the door in the back to unseal it and acquire the Magus Sisters.