The Magus sisters are three Aeons who are sisters and help each other out in battle. They happen to be the most powerful Aeon in the entire game, and dish out tons of damage with their limit break. You'll for sure not want to miss up the opportunity to obtain them!
The first thing you have to do is get all the other Aeons. Which will mean you'll need both Yojimbo and Anima before you can get the Magus Sisters. If you have aquired those Aeons, head to the monster Arena. You'll have to now capture one of every type of fiend located on Mt. Gagazet. These might take a bit to do, but just remember, the reward is definitely worth it. In exchange for your efforts, you'll be rewarded with the Blossom Crown for capturing the fiends.
Afterwards, head to the Remiem Temple. You can get there while riding a chocobo. Just go to the Lower area of the Calm Lands near the exit of Macalania Woods. Around the area a little to the right you should see a chocobo feather on the ground. Press the X button and the chocobo you're riding should fly to another area which will allow access to the temple.
When you enter the temple you will notice that Belgemine is there, the woman who Yuna fought against several time in Aeon battles. (This, however, was optional during the game.) You'll need to defeat every Aeon she possesses, which are basically just exact clones of all of Yuna's. After doing so, you can head to the back and use the Flower Scepter (Which she will give you for defeating Aeons.) and Blossom Crown on the door in the back. After using it, you'll aquire the Magus Sisters. Congrats!
Since there are three of them, their commands are a bit different than normal. Below is a list of the commands and what they do.
Do What You Will - The sisters will do whatever they want in battle.
Fight! - The Magus Sisters will attack regularly.
Go! Go! - They'll perform their various unique attacks or use magic spells.
One More Time - This will command the Magus Sisters to repeat the very last action. This could be useful.
Defense! - They will cast supportive magic on each other.
Help each other! - They will use healing magic.
Dismiss - They Will be dismissed. Works the same as other commands.
Combine Powers! - They use their overdrive.
Unique Attack - Camisade, Razzia, Passado The Magus Sisters have three different unique attacks - a different one for each sister. Passado seems to be the most powerful, shooting several stingers at an enemy reducing their HP. It seems it has some kind of percentage effect, meaning it will only take off a certain percentage of HP compared to the target's maximum. Razzia is just a general attack while Camisade is also a general attack.
Overdrive - Delta This is probably one overdrive you'll want to use quite often. It's a long sequence, but when it's over MASSIVE amounts of damage will be done to all enemies onscreen.