wp: Display immediate children or siblings as a menu. [WordPress]

gistfile1.php

<?php
/**
 * Get related menu
 *
 * Displays immediate children or siblings
 *
 * @param boolean $echo echo or return result
 * @return str $r
 */
function prefix_related_menu( $echo = true ) {

    global $post;

    $r = '';

    // bail if we don't have any ids to work with
    if( is_null( $post ) )
        return $r;

    // get children
    $pages = wp_list_pages( 'title_li=&child_of=' . $post->ID . '&echo=0' );

    // get siblings
    if( ! $pages && $post->post_parent ) {
        $pages = wp_list_pages( 'title_li=&child_of=' . $post->post_parent . '&depth=1&echo=0' );
    }

    // output menu
    if( $pages ) {

        $r .= '<div class="widget">' . "\n";
        $r .= '    <h3 class="widget__heading">Related</h3>' . "\n";
        $r .= '    <ul class="menu related-menu">' . "\n";
        $r .= '    ' . $pages . "\n";
        $r .= '    </ul>' . "\n";
        $r .= '</div>' . "\n";

    }

    if( $echo ) :
        echo $r;
    else :
        return $r;
    endif;

}