Oh man... Since my shop was quite customized, i couldn't use the modified files but had to do it all manually. So, at the end, i figured out that my includes/modules/sideboxes/categories.php was almost an empty file nad had to completly replace it. The same thing was with includes/templates/template_default/sideboxes/tpl_categories.php
After replacing those files and modifying all other files, I got this working.

Is there a way to filter EZpages or define_pages for different sites?

@Little: What sidebox are you using to display categories (search by category)? Make sure you're using categories.php since that's the file you previously edited. Also, your search by brand doesn't work.