<?php
$ancestor_id=get_main_page_id();
$descendants = get_pages(array('child_of' => $ancestor_id));
$incl = "";

foreach ($descendants as $page) {
   if (($page->post_parent == $ancestor_id) ||
       ($page->post_parent == $post->post_parent) ||
       ($page->post_parent == $post->ID))
   {
      $incl .= $page->ID . ",";
   }
}?>

<ul id="sub_menu">
   <?php wp_list_pages(array("child_of" => $ancestor_id, "include" => $incl, "link_before" => "", "title_li" => "", "sort_column" => "menu_order"));?>
</ul>