Load parent pages when there are no child pages

Question

I use the following code to load subpages in a sidebar on all pages:

<?php
                 $children = wp_list_pages('title_li=&child_of='.$post->ID.'&echo=0&depth=1');
                  if ($children) { ?>
                  <ul id="three-menu">
                  <?php echo $children; ?>
                  </ul>
                  <?php } ?>

It works well on all pages with sub pages or children. But what if I want this sidebar with children pages to load with parents]when there are no children? What code to add to another if or elseif? Basically I need to add a No children? Then loads parent pages..

Update:

At http://www.fldtrace.com/wordpress/how-to-display-parent-page-title-in-wordpress I found this code to load the parent page link:

<?php

$parent_title = get_the_title($post->post_parent);?>
<a href="<?php echo get_permalink($post->post_parent) ?>"><?php echo $parent_title;?></a>

Which is great, but I need to list all parent pages, which are sub pages of the grand parent...

Answer 1

possible code:

<?php                  
$children = wp_list_pages('title_li=&child_of='.$post->ID.'&echo=0&depth=1');                   
    if ($children) { ?>                   
        <ul id="three-menu">                   
        <?php echo $children; ?>                   
        </ul>                   
    <?php } //ends (if($children)//
    elseif($post->post_parent) { //if no children, try to get parent page and show siblings pages including the page itself
    $siblings = wp_list_pages('title_li=&child_of='.$post->post_parent.'&echo=0&depth=1');                  
        if ($siblings) { ?>                   
            <ul id="three-menu">                   
            <?php echo $siblings; ?>                   
            </ul> 
        <?php } //ends if($siblings)// ?>                  
    <?php } else { //optional: if no children and if no parent, then show all top level pages
    $pagelist = wp_list_pages('title_li=&echo=0&depth=1');                   
        if ($pagelist) { ?>                   
            <ul id="three-menu">                   
            <?php echo $pagelist; ?>                   
            </ul>
        <?php } //ends if($pagelist)// ?>
    <?php } ?>  

contains a lot of redundant coding to keep the structure recognizable simple.