find your hosts file and edit it to include the line:
127.0.0.1
www.friendster.com
This will redirect any attempt to access friendster to the loopback address, which goes right back to your PC. Since your PC normaly isnt serving a webpage, this will result in your sister getting told that she cannot connect to
www.friendster.com.
But, is it really worth it? Whats wrong with the site?