My brother wants me to backup the data in his SIM card. How can I copy this to my computer without messing up my own contacts?
Create a separate Google account for your brother's account. Import your brother's SIM's contacts to that Google account.
OK- here's the idea I have:
You can either sync the new contacts to the new Google account or export them to a .vcf on your SD card.
This is just a rough idea of how you can do things. The end process is yours to tweak to your liking and needs.