Replace $parent in child_of argument of wp_list_pages by parent->ID.
wp_list_pages needs the post id instead of post object.
<?php
global $post;
$direct_parent = $post->post_parent;
$parent = $direct_parent->post_parent;
wp_list_pages( array(
'child_of' => $parent,
'title_li' => false,
'depth' => 1
) );
?>
