// Alter the related blog posts query to grab items from first category
add_filter( 'wpex_blog_post_related_query_args', function ( $args ) {
// Get first category
$cats = wp_get_post_terms( get_the_ID(), 'category' );
// Get from 1st category only
$cats['category__in'] = array( $cats[0]->term_id );
// Return args
return $args;
} );