Well, if I wanted to transfer a boatload of music to a computer owned by a relative, I'd just hook it up to my LAN and copy the folders over, it's the easiest way.
If you *need* to use the iPod, mount it as a disk, copy the stuff over (if you have enough space to essentially mirror your collection on the thing), and see if it can mount without iTunes on your sister's computer.
It's a pain in the ass, I know, but the iPod's one-way communication is probably crucial to Apple's deals for song downloads in iTunes.