It sounds like your brothers computer also has itunes installed and your ipod is trying to auto sync with his music library.
All you need to do is just turn off your ipod auto sync when connneted to your computer so you can hook it up to any computer without it trying to erase anything. The auto sync preference is stored on the ipod, not the computer, so once set, it will stay when connecting to another computer. Just make sure you always update your music manually, and not using the itunes auto sync.
The one and only issue with connecting to other computers has to do with music purchased through itunes store. You have to authorize the computer to play any itunes store music by logging into itunes store on each computer. And you can only have 5 computers authorized at once.
Music cut from cd's or downloaded from sites without copy protection can be put on your ipod from any computer with itunes. And its even possible to load music from itunes store by cutting it to a cd first and then ripping it back to the computer as an mp3.
Now if you want to take the music you got from another computer and put it onto your computer via your ipod, that is a much harder issue. The ipods store the music in a very random structure so you have to do alot of searching by song name. And thats after you get the ipod to show up as a external hard drive on your computer.