page siblings

<?php global $post; //not neccessary if used in the loop
$parent = $post->post_parent;
if( $parent ) :
    $siblings = get_pages( 'child_of=' . $parent . '&parent=' . $parent . '&exclude=' . $post->ID);
    if( $siblings ) foreach( $siblings as $sibling ) :
        //start of whatever you need to output//
        echo $sibling->post_title;
        echo get_the_post_thumbnail($sibling->ID,'medium');
        //end of whatever you need to output//
    endforeach;
endif; //ends if( $parent ) //
?>