[Plugin: Child Page Navigation] Enhancement: Show parent page if needed ;-)

Good work with your widget. In my case, I needed to display the parent page under some cirumstances, so I made a quick enhancement:

<?php
/*
    Plugin Name: Child Page Navigation
    Plugin URI: https://www.ads-software.com/extend/plugins/child-page-navigation/
    Description: Provides a theme-independent widget to display the child pages of the current page.
    Author: ITS Alaska
    Author URI: https://ITSCanFixThat.com/
    Version: 1.2.1

    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <https://www.gnu.org/licenses/>.
*/

    class theme_navigation extends WP_Widget {
        function theme_navigation() {
            parent::WP_Widget('theme_navigation', 'Child Page Navigation', array('description' => '', 'class' => 'theme-navigation'));
        }
        function form($instance) {
            $default =     array( 'title' => __('Navigation'), 'show_parent' => 0 );
            $instance = wp_parse_args( (array) $instance, $default );
            $field_id = $this->get_field_id('title');
            $field_name = $this->get_field_name('title');
            echo "\r\n".'<p><label for="'.$field_id.'">'.__('Title').': <input type="text" class="widefat" id="'.$field_id.'" name="'.$field_name.'" value="'.attribute_escape( $instance['title'] ).'" /><label></p>';
			echo "\r\n".'<p><input class="checkbox" type="checkbox" value="1" ';
			checked( $instance['show_parent'], 1 );
			echo ' id="'.$this->get_field_id( 'show_parent' ).'" name="'.$this->get_field_name( 'show_parent' ).'" />
			<label for="'.$this->get_field_id( 'show_parent' ).'">Show parent page?</label></p>';
            $field_id = $this->get_field_id('page');
        }
        function update($new_instance, $old_instance) {
            $instance = $old_instance;
            $instance['title'] = strip_tags($new_instance['title']);
			$instance['show_parent'] = $new_instance['show_parent'];
            return $instance;
        }
        function widget($args, $instance) {
            extract($args, EXTR_SKIP);
            global $post;
            $pages = get_pages(array(
                'child_of' => $post->ID,
                'parent' => $post->ID,
                'sort_column' => 'menu_order'
            ));

			// Chek if Parent should be displayed
			$show_parent = isset( $instance['show_parent'] ) ? $instance['show_parent'] : false;

            // Check if the current page has a parent
            if ($post -> post_parent) {
                // Get sibling pages
                $siblings = get_pages(array(
                    'child_of' => $post -> post_parent,
                    'parent' => $post -> post_parent,
                    'sort_column' => 'menu_order'
                ));
            }

            if(count($pages)){
                echo $before_widget;
                $title = empty($instance['title']) ? '' : apply_filters('widget_title', $instance['title']);
                if ( !empty( $title ) ) { echo $before_title . $title . $after_title; }
                echo "<ul>";
				if ( $show_parent  == 1){
					echo "<li><a href='".get_permalink($page->post_parent)."'>".get_the_title($page->post_parent)."</a></li>";
				}
                foreach($pages as $page){
                    echo "<li><a href='".get_permalink($page->ID)."'>".get_the_title($page->ID)."</a></li>";
                }
                echo "</ul>";
                echo $after_widget;
            }elseif($post -> post_parent) { // Show the sibling pages if there are no children
                if(count($siblings)) {
                    echo $before_widget;
                    $title = empty($instance['title']) ? '' : apply_filters('widget_title', $instance['title']);
                    if ( !empty( $title ) ) { echo $before_title . $title . $after_title; }
                    echo "<ul>";
					if ( $show_parent == 1){
						echo "<li><a href='".get_permalink($post -> post_parent)."'>".get_the_title($post -> post_parent)."</a></li>";
					}
                    foreach($siblings as $page){
                        echo "<li><a href='".get_permalink($page->ID)."'>".get_the_title($page->ID)."</a></li>";
                    }
                    echo "</ul>";
                    echo $after_widget;
                }
            }
        }
    }
    function register_theme_navigation_widget(){
        register_widget('theme_navigation');
    }
    add_action('widgets_init','register_theme_navigation_widget');

?>

Just in case someone could make use of this.