Welcome tiago! and this is where you post your questions and here is how to get the Magus Sisters.
First of all, get Yojimbo and Anima then go to the Remiem Temple in the Calm Lands to get there, mount a Chocobo, go to the entrance of the area and to the right examine the Chocobo Feather to jump to the temple entrance and enter and defeat Belgemine's Aeons(Only required ones are Valefor, Ifrit, Ixion, Shiva, and Bahamut)You receive the Flower Scepter after Bahamut. Then go to Mt. Gagazet and capture the following fiends:Bandersnatch, Bashura, Grat, Behemoth, Grenade, Dark Flan, Ahriman,Mandragora, Achelous, Splasher, Maelspike, and Grendel.
After that go back to the monster arena and you receive the Blossom Crown from the Monster Arena guy. Then go to Remiem Temple and use the Blossom Crown and the Flower Scepter to break the seals and you will receive the Magus Sisters!