It depends on the machine and operating system you're running. In OSX this is done through System Preferences. Alternatively you could create two different iTunes libraries by holding OPTION as you open iTunes. This'll give you the ability to create a new library or open an alternative one (for when you'll be switching between them). However, this will probably be more work than it's worth and will most likely result in a lot of double-ups. Instead, I'd recommend using playlists to sync your iPods. That'll give you a selection of the library you already have, plus the ability to weed out all your Mum's music. 🙂