The Magus Sisters are the last of the optional Aeons that you can obtain. You have to obtain all of the other Aeons before you can obtain them.
Check out the Magus Sisters side quest section for more information on how to obtain them:
The Magus Sisters are difficult to control. Rather than providing commands of which attack to do and when, Yuna will only shout suggestions such as “Help each Other!”, “Go, Go, Go!”, “Fight!”, etc. Following this, Cindy, Sandy and Mindy will decide the most appropriate action.
Cindy is the big, fat sister modelled after a lady bug. She is primarily responsible for casting preventative and curative spells, such as 
Reflect and Cura.
Mindy is the tiny little bug modelled after a wasp. She casts high level magic spells.
Sandy is the tall bug modelled after a walking stick. She primarily does regular attacks and tends to be the strongest of the three sisters when you summon them.
The Magus Sisters will only use their Overdrive (Delta Attack) when all three of them have filled their Overdrive meters. They each have a special attack: Cindy has Camisade, Mindy has Passado and Sandy has Razzia. Despite the difficulty in controlling them, the Magus Sisters are capable of breaking the 9,999 damage limit as soon as you obtain them which makes them fairly powerful to use in battle.
