<?php // paste this in functions.php
function wpe_highest_ancestor(){
    global $post;
    $page_ancestors = end($post->ancestors);
    if(!empty($page_ancestors)){
        $child_of = $page_ancestors;
    }else{
        $child_of = $post->ID;
    }
    return $child_of;
}
?>

This is for the template
-------------------------------
<ul class="sidebar">
  <?php wp_list_pages('child_of=' . wpe_highest_ancestor()); ?>
</ul>

If you want to have the name of the top-level page in the sidebar, you can use the following code:
<ul class="sidebar">
  <?php
  $child_of = wpe_highest_ancestor();
  wp_list_pages('child_of='.$child_of.'&title_li=<h5>'.esc_attr(get_the_title($child_of)).'</h5>');
?>
</ul>

Wordpress : Display Always All Subpages in Sidebar

wordpress, Development

PHP