The last of the secret Aeons is the eccentric trio known as the "Magus Sisters." They act on their own and they may take the occasional break, but they are very powerful and can unleash some nasty attacks when properly motivated!
Obtain Yojimbo and Anima (see Related eHows, "How to Obtain the Secret Aeons in Final Fantasy X, Part 1" and "Part 2").
Head to Remiem Temple in the Calm Lands. If you are unsure how to get there, rent a Chocobo. Head to the entrance to Macalania, but keep going across to the right. You should see a Chocobo feather on the ground next to a ruined bridge. Prompt your Chocobo to fly across the bridge and continue on through the canyon.
Challenge and defeat all of Belgemine's Aeons. You needn't do this all at once. You will receive the Flower Scepter after you defeat Bahamut.
Head to Mt. Gagazet and capture at least one of every fiend there (Bandersnatch, Ahriman, Dark Flan, Grenade, Grat, Grendel, Bashura, Mandragora, Behemoth, Splasher, Achelous and Maelspike).
Return to the Arena and collect the Blossom Crown from the Arena Master.
Take the Crown and Scepter back to Remiem Temple to get the Magus Sisters!
Have them "Do as they will" to let them decide what they do. Could be beneficial, could be something utterly useless. It's a gamble, but it can pay off.
Tell them "Fight!" to have them use a physical or special attack.
"Help each other!" causes them to heal or buff one another.
"Go, go!" spurs them to attack with magic and special abilities.
"Offense!" makes them use attack magic.
"Defense!" encourages them to use defensive magic.
"What happened to you?" will cause them to use the Drain spell on an enemy.
"Combine Powers!" will have them perform their Delta Attack overdrive. You've got to see this, it's nifty!
"Dismiss." Guess what this does! No surprise, it sends them away.