How do you beat sin when you are on the airship, I have used an aeon to..

It has 140,000hp. Try to equip armour with confusion, stone and zombie proof/ward. Cast haste/hastega a.s.a.p and keep hiting it with lulu's magic, wakka's ball and other long range attacks. when it moves forward twice it should be close enough to attack, now get auron to use armour/mental break and use your 3 strongest attackers and keep hiting it while curing. If you havent beaten it before ure last move quickly switch to yuna and summon bahamut (with full overdrive gauge) and use mega flare if its not dead now your people need some training.

If I were you I would concentrate on getting Yojimbo, Anima and the Magus Sisters. They are not that hard to get and with these you will fly through the rest of the game. The Magus sisters will kill sin him easly.

No you shouldn't use aeons to block the final attack because it wont work so you have to just attach him with strong attach dont charge or get the secret aeons.

The final attack will wipe out everything, there is not defense against it. Basically it's a timed battle, you have to kill it before it does it's final attack.
To beat it you use long range attacks (like Wakka's blitzball, any magic and any overdrives that will reach that far, including Aeons overdrives) until it gets close enough to use close range attacks. Then use your strongest attacking characters to hack away as quickly as possible, and use the rest of your characters and Aeons overdrives. Basically throw everything you have at it, as quickly as possible.