show all post in their parent category may be work?
<?php
$currentCatId = get_query_var('cat');
$currentCatParent = get_term_by('id', $currentCatId ,'category');
$currentCatParentId = $currentCatParent->parent;
$my_query = new WP_Query( 'cat=".$currentCatParentId );
print_r($my_query);
if ($my_query->have_posts()) : while ($my_query->have_posts()) : $my_query->the_post();
// Do your stuff
endwhile;
endif;
?>