To location of the Magus Sisters is in the Remiem Temple, in order to acquire them need to have all the other aeons (including Anima and Yojimbo), defeated and captured all 10 species of animal at Mt. Gagazet (this gets u the Blossom Crown from the Monster Arena) and defeated the first 5 of Belgemine's aeons (givng u the Flower Sceptre)

Next Q:
What are the maximum stats that each character can obtain? (Party members like Tidus, Auron, etc not Aeons).