Page's siblings with thumbs

I make some navigation with thumbnails. I’ve made already such navigation for parent page, but struggle with getting the thumbs of sibblings while exploring one of children. Idea is that when I explore one of children I get in the navigation all siblings.

So far I found this:

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

For the navigation for parents’ page I use this:

<?php
$child_pages = $wpdb->get_results("SELECT *    FROM $wpdb->posts WHERE post_parent = ".$post->ID."    AND post_type = 'page' ORDER BY menu_order", 'OBJECT');    ?>
<?php if ( $child_pages ) : foreach ( $child_pages as $pageChild ) : setup_postdata( $pageChild ); ?>
<div class="thumb">
<div class="thumb_img"><a href="<?php echo  get_permalink($pageChild->ID); ?>" rel="bookmark" title="<?php echo $pageChild->post_title; ?>">
 <?php echo get_the_post_thumbnail($pageChild->ID, 'medium'); ?></a>
<h2><?php echo $pageChild->post_title; ?></h2>
 </div>

</div>
<?php endforeach; endif;
?>

How can I modify ‘echo $children;’ to get the thumbnails of children?