Add Parent to Subpage List

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

 if ($children) { ?>
   <ul class="subpages">
     <?php echo $children; ?>
   </ul>
 <?php } ?>

Try that. Just have to include the parent much like you do on the child pages, but using the current $post-ID.