I am so sorry to hear about your brother. I hope you and your family get through it as best you can. (sorry, I'm a bit crap at condolences).
As for the mac, just boot from the install cd (insert the disc and restart while holding down "c" on the keyboard), quit the installer and go to "disc utility" (it's in one of the pull down menus in the menubar at the top), highlight the harddrive in the sidebar of the application, choose "Erase" in the button options towards the top of the window, and click "security options". If you're ultra worried, choose "35 pass erase", but be prepared to leave it overnight or longer, it will take a while. 7 pass or even just zero out should suffice.
Again, my sincerest condolences.