I came up with this code:
<?php
if ($post->post_parent)
$children = wp_list_pages("depth=-1&title_li=&child_of=" . $post->post_parent . "&echo=0");
else
$children = wp_list_pages("depth=-3&title_li=&child_of=" . $post->post_parent->ID . "&echo=0");
if ($children) {
?>
<ul>
<?php echo $children; ?>
</ul>
<?php } ?>
But if I visit domain.com/page/subpage/subpageForSubpage, it shows all of the subpages of “subpage”. Hope you guys understand ??