The Magus Sisters
The Magus Sisters are the third and final optional Aeon players are able to acquire for Yuna in Final Fantasy X. In order to obtain this Aeon you'll first need to have already acquired the Yojimbo and Anima optional Aeons and collect two items. The first of which is the Blossom Crown which is a reward for capturing each of the different fiends from Mount Gagazet. More information on that can be found in my Monster Arena Guide. The second item we get later in this guide.
Once you're ready you can find the Magus Sisters at Remiem Temple located in a room behind Belgemine, whom is the summoner you can periodically challenge throughout the game in different locations. When you talk to her after you go through her dialogue she'll battle you in a Summoner's battle, Aeon vs Aeon.
If you forgot where Remiem Temple is, it's just off of the Calm Lands and you need a Chocobo to reach it. It's the same location you had to go for the Cloudy Mirror, part of the Celestial Weapons quest. A picture of the temple can be found in my screenshot below.
In order to obtain the second item you need for the Magus Sisters you'll need to beat each of Belgemine's Aeons, which should be extremely easy for you if you've level up appropriately. I used Bahamut for each of the battles and beat all her Aeons in one shot. Once you beat her Bahamut you'll be rewarded with the Flower Scepter which is the second item required for unlocking the seal to the Magus Sisters.
You still have to beat the optional Aeons too if you wish to acquire Yuna's Moon Sigil but first we need to grab the Magus Sisters. Behind Belgemine you'll find a door with a purple seal, as seen in my screenshot below. This door will require you to use both the Flower Scepter and the Blossom Crown to unlock it. Use both on it and you'll be granted access to the Chamber of the Fayth.
Inside the Chamber you'll have a short cutscene where you see each of the Magus Sisters while they were still human - after you'll have the opportunity to name them like you do with all the Aeons you get. These guys aren't really that useful as far as Aeons go if you ask me since you don't really get full control over them during a battle. Once you use them you'll notice what I mean.
Either way, once you've acquired these Aeons go back out to Belgemine and finish fighting her. Once you defeat her Yojimbo, Anima and Magus Sisters she will reward you with Yuna's Moon Sigil which is of course used in powering up her Celestial Weapon, Nirvana.