HIDDEN AEONS

Getting Yojimbo is quite easy. Enter the Cave of the Stolen Fayth (when you exit the Calm Lands, descend into the valley instead of going up the path to Mt. Gagazet). The spirit of a dead summoner will appear within, and she'll call upon Yojimbo to do battle with you. Defeat him, then use the teleport pad at the dead end to enter the Chamber of the Fayth (after getting the treasures to the left and right first, that is).

When you speak to Yojimbo, you'll be given a choice of what to say to persuade him to aid you. Choose the option "To defeat the most powerful of enemies" and he'll offer the lowest price, 250,000 Gil (yes, you have to pay him in cold, hard Gil to get him to join you. He must have been a mercenary or something). If you choose the other options, he'll ask for either 270,000 Gil or 300,000 Gil, depending on what you chose. You don't have to pay what he asks immediately; you can bargain for a bit and try to make him lower his price. Don't offer him less than half of what he asks though, or he'll refuse to negotiate at all. It's a good idea to offer him about 70% of whatever he's asking for, and he'll lower his price somewhat. You can do this five times before he refuses to negotiate. At any time, if you offer him triple his price, you'll receive two Teleport Spheres in addition to his services (but it's really not worth it, in my opinion).

Anima

Anima is a bit more troublesome to get. In each of the Cloisters of Trials you went through to get Valefor, Ifrit, Ixion, Shiva and Bahamut, there is one hidden treasure you can get (the ones you got using Destruction Spheres). So you should have five of them now. If not, you need to go and get whatever you've missed first. However, note that if you missed the hidden treasure in Bevelle Temple, there's no way you can go back to get it, unfortunately, so you'll be unable to get not just Anima, but the Magus Sisters as well. That means you can't get the Moon Sigil for Yuna's Nirvana either. If you missed any of the other four hidden treasures, you can still go back to get them. The hidden treasures from the Cloisters of Trials are as follows:

Cloister of Trials - Besaid Temple		Rod of Wisdom
Cloister of Trials - Kilika Temple		Red Armlet
Cloister of Trials - Djose Temple		Magic Sphere
Cloister of Trials - Macalania Temple		Luck Sphere
Cloister of Trials - Bevelle Temple		Knight Lance
Cloister of Trials - Zanarkand Dome		Magistral Rod

You'll get the last hidden treasure from the Zanarkand Dome Cloister of Trials later, so don't worry about it for now. Use your airship to locate the Baaj Temple (it's a hidden area), go there and defeat Geosgaeno before entering the temple via the underwater entrance. On a sidenote, you can get Lulu's Onion Knight here as well.

In the temple, you'll find six statues. After you've examined each of them, five of the statues will be activated by the five hidden treasures you have, so now you need to return to the Zanarkand Dome Cloister of Trials where there'll be a new puzzle for you to solve in order to get the last hidden treasure. Once you've gotten that, go back to the Baaj Temple to activate the last statue, and you'll be able to enter the Chamber of the Fayth and obtain Anima.

The Magus Sisters

Now for the last of the hidden aeons! First, you need to have the Blossom Crown (which you get from the monster trainer for unlocking the battle against Catoblepas, by capturing at least 1 specimen of each type of monster (total of 12 types) on Mt. Gagazet) and the Flower Scepter (which you get the first time you defeat Bahamut in the aeon battles at the Remiem Temple). Next, you must have obtained all the other aeons, including Yojimbo and Anima. Now, after you've defeated each of the aeons at the Remiem Temple at least once, go to the door at the back of the temple and use the Blossom Crown and the Flower Scepter on it to unlock the seal, hereby acquiring the Magus Sisters. That's all there is!