How can I backup contacts on a SIM card to my computer? 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:


Back up all your existing contacts to SD card. Open your Contacts App and press MENU > IMPORT/EXPORT > EXPORT TO SD CARD. A .vcf file will be created with all your existing contacts. Using My Files or Astro or a file manager of choice, save / copy this backup file to another location as well.
Optional Step - You can create a separate Google account for your brother so his imported contacts are pushed/synced there.
Turn on Airplane Mode - this way there's no chance of any changes being made to your Google Account Contacts (added safety).
Wipe all your contacts - from within the Contacts app, press MENU > DELETE and choose ALL CONTACTS. (This is also rather optional, depending on whether you wish to sync to a Google account or not).
Turn your phone off and insert your brother's SIM. Turn your phone on.
You currently have no contacts. From within your CONTACTS app, press MENU > IMPORT / EXPORT and choose IMPORT FROM SIM - it'll ask where the contacts should be imported to - PHONE / list of Google Accounts. Depending on your process, either choose the PHONE or the new Google account and you ought to be done.


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.