None of the FF games are related in anyway, besides of course FFX and FFX-2. You wont need to have played FFX though to get into X-2.
Both games take place in the same world, but they are very much different, you will visit a lot of the locations from FFX, but just atmopshere in X-2 is much a lot different. For example, in FFX, the NPC's are sad, depressed and afraid because of this magical monster called Sin, after FFX however Sin is defeated and that is where FFX-2 starts off. So everyone in FFX-2 is happy and wanting to adventure, it is something they have never done for many generations. You'll get a good fill in anyways into what happened in FFX from playing FFX-2. The NPC's make it a point to always remember the past, the story line is FFX and X-2 is the deepest story line of any FF game so far. It isnt something I can really explain, you gotta play it for yourself to understand.