Thank you very much Jonathan,
your response helped me a lot.
Here is the code i’ve added in function.php and it runs fine:
function my_relationship_query( $args, $field, $post )
{
$args["meta_key"]="show_in_news";
$args["meta_value"]="1";
return $args;
}
// filter for a specific field based on it's name
add_filter('acf/fields/relationship/query/name=related_articles', 'my_relationship_query', 10, 3);