Get's the next/previous page (or hierarchical cpt) with the same parent as the current post.

previous-next-sibling.php

<?php
/*
 * Gets IDs of the previous and next siblings (pages with the same parent as current post).
 * Then uses wpse5422_display_prev_next to display links to them 
 * Must be used inside the loop.
 *
 * This is my solution to: http://wordpress.stackexchange.com/a/54463/9364
 */
function wpse5422_the_page_siblings(){
    $post_id = get_the_ID();
    $parent_id = wp_get_post_parent_id( $post_ID );
    $post_type = get_post_type($post_id);

    $sibling_list = get_pages(array(
         'sort_column'=>'menu_order',
         'sort_order' =>'asc',
         'child_of' =>$parent_id,
         'post_type'=> $post_type
    ));

    if( !$sibling_list || is_wp_error($sibling_list) )
        return false;

    $pages = array();
    foreach ($sibling_list as $sibling ) {
         $pages[] = $sibling->ID;
     }

    $current = array_search($post_id, $pages);
    $prevID = isset($pages[$current-1]) ? $pages[$current-1] : false;
    $nextID = isset($pages[$current+1]) ? $pages[$current+1] : false;

    echo wpse5422_display_prev_next($prevID, $nextID);
 }


/*
 * Generates and returns html for displaying 'previous' and 'next' links given the post IDs.
 *
 * @param (int) $prevID - ID of the post that is 'previous'
 * @param (int) $nextID- ID of the post that is 'next'
 */
 function wpse5422_display_prev_next($prevID=false, $nextID=false){
    if( empty($prevID) && empty($nextID) )
       return false;

    $html = '<div class="navigation">';

    if( !empty($prevID) ){
         $html .= '<div class="alignleft">';
         $html .= '<a href="'.get_permalink($prevID).'">Previous</a>';
         $html .= '</div>';
     }

    if( !empty($nextID) ){
         $html .= '<div class="alignright">';
         $html .= '<a href="'.get_permalink($nextID).'">Next</a>';
         $html .= '</div>';
     }

    $html .= '</div><!-- .navigation -->';

    return $html;
}
?>