If you already know the Actualiteiten cat_ID as it is quite specific, lets say it was category 22!
<?php
global $cat;
$counter = ($cat == 22) ? 0 : 1;
or if you want to make it easy to read:
<?php
global $cat;
$curr_cat = get_category($cat);
$counter = ($curr_cat->slug == 'actualiteiten' ) ? 0 : 1;
If the category is a match then the counter is ? 0 (false) else : it is 1 (true)
If you make the post not sticky how are you going to make it the first post all the time, when you add new posts to the category?
One way would be to set the date and time of the post you want first to be the latest.
Also consider Template Hierarchy, you can copy category.php and name it category-{slug}.php so category-actualiteiten.php and add in code to return the main post first, no need for counters.
HTH
David