Show a parent's children pages. On children pages, show siblings.
<?php
if ( $post->post_parent ) {
$children = wp_list_pages( array(
'title_li' => '',
'child_of' => $post->post_parent,
'echo' => 0
) );
} else {
$children = wp_list_pages( array(
'title_li' => '',
'child_of' => $post->ID,
'echo' => 0
) );
}
if ( $children ) : ?>
<h3 class="blue">
<?php echo get_the_title( $post->post_parent ); ?>
</h3>
<ul class="sidebox-list">
<?php echo $children; ?>
</ul>
<?php endif; ?>
<?php
$args = array(
'post_type' => 'page',
'posts_per_page' => -1,
'post_parent' => $post->ID,
'order' => 'ASC',
'orderby' => 'menu_order'
);
$parent = new WP_Query( $args );
if ( $parent->have_posts() ) : ?>