Following code will display all the subpages of same parent page. also displays the subpages of current page if it is not a subpage.

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

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

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

<?php } ?>