List subpages even if on a subpage

sidebar.php

<!--display subpages if any -->
<?php
  if($post->post_parent)
  $children = wp_list_pages("title_li=&child_of=".$post->post_parent."&echo=0");
  else
  $children = wp_list_pages("title_li=&child_of=".$post->ID."&echo=0");
  if ($children) { ?>
  <div class="sidebar_widget subpages">
      <ul>
  <?php echo $children; ?>
  </ul>
    </div>
<?php } ?>
<!-- end .subpages -->

http://codex.wordpress.org/Function_Reference/wp_list_pages#List_Sub-Pages