This function will do the trick, it displays the child pages of the current page, but if there are no child pages, it displays the child pages of the parent page, and a link back to the parent page.
/**
* @brief: Shows the subpages of the current page, or
* the adjacent sibling pages.
**/
function show_subpages(){
global $post;
$subpages = wp_list_pages( array(
'echo'=>0,
'title_li'=>'',
'depth'=>2,
'child_of'=> ( $post->post_parent == 0 ? $post->ID : $post->post_parent)
));
if ( !empty($subpages) ) {
if ( $post->post_parent != 0 ) {
echo '<p class="parent-link"><em>'. __('Back to') .' <a href="'. get_permalink($post->post_parent) .'">'. get_the_title($post->post_parent) .'</em></a><p>';
}
echo '<ul>';
echo $subpages;
echo '</ul>';
} else {
echo 'no subpages';
}
}
You may need to place this in the main loop