A useful code snippet I thought I’d share
<div id="subpages">
<h3>Related Pages</h3>
<?php
// If CHILD_OF is not NULL, then this page has a parent
// Therefore, list siblings i.e. subpages of this page's parent
if($post->post_parent){
wp_list_pages('title_li=&include='.$post->post_parent);
wp_list_pages('title_li=&child_of='.$post->post_parent);
}
// If CHILD_OF is zero, this is a top level page, so list subpages only.
else{
wp_list_pages('title_li=&include='.$post->ID);
wp_list_pages('title_li=&child_of='.$post->ID);
}
?>
</div>