Shortcode method to generate an unordered HTML list of either child or sibling links, relative to the current post

gistfile1.php

<?php

/**
 * Render an unordered HTML list of related page links
 *
 * @return string
 */
function renderRelatedLinks($atts)
{
    global $post, $wpdb;

    extract(shortcode_atts(
        array(
            'type' => 'child',
        ),
        $atts
    ));

    if ( ! isset($post->ID) ) {
        return null;
    }

    switch ($type) {
        case 'child':
            $parent = $post->ID;
            break;
        case 'sibling':
            $parent = $post->post_parent;
            break;
        default:
            $parent = null;
            break;
    }

    if ( is_null($parent) ) {
        return null;
    }

    $sql = sprintf(
        '   SELECT      `ID`,
                        `post_title`,
                        `menu_order`
            FROM        `%s`
            WHERE       `post_type` = "page"
            AND         `post_parent` = %d
            ORDER BY    `menu_order`, `post_title`',
        $wpdb->posts,
        $parent
    );

    if ( $siblings = $wpdb->get_results($sql) ) {
        $html = null;

        foreach ($siblings as $sibling) {
            $current = ($sibling->ID == $post->ID);

            $html .= sprintf(
                '<li class="%s"><a href="%s">%s</a></li>',
                $current ? 'current' : null,
                get_permalink($sibling->ID),
                $sibling->post_title
            );
        }

        if ( ! is_null($html) ) {
            $html = sprintf('<ul>%s</ul>', $html);
        }

        return $html;
    }

    return null;
}

add_shortcode('sc_related_links', 'renderRelatedLinks');