Modifing sidebar menu – wp list pages

Found this code on codex for sidebar menu. Works almost perfect, except of one thing: When visiting a child page, just the children, and descendant pages, of that parent, are listed.
All other top level pages disappear from the menu, I want them to be shown.

<?php
  if($post->post_parent) {
  $children = wp_list_pages("title_li=&child_of=".$post->post_parent."&echo=0");
  $titlenamer = get_the_title($post->post_parent);
  }

  else {
  $children = wp_list_pages("title_li=&child_of=".$post->ID."&echo=0");
  $titlenamer = get_the_title($post->ID);
  }
  if ($children) { ?>

  <h2> <?php echo $titlenamer; ?> </h2>
  <ul>
  <?php echo $children; ?>
  </ul>

<?php } ?>