Listar paginas irmãs.

page.php

<?php
if ( $post->post_parent ) {
    $args = array(
        'hierarchical' => 1,
        'exclude' => $post->ID,
        'child_of' => $post->post_parent,
        'post_type' => 'page',
        'post_status' => 'publish'
    );
} else {
    $args = array(
        'hierarchical' => 1,
        'exclude' => $post->ID,
        'child_of' => $post->ID,
        'post_type' => 'page',
        'post_status' => 'publish'
    );
}

$my_pages = get_pages( $args );

echo '<ul>';
foreach ( $my_pages as $my_page ) {
    echo sprintf( '<li><a href="%1$s" title="%2$s">%2$s</a></li>', get_permalink( $my_page->ID ), $my_page->post_title );
}
echo '</ul>';