Add this code in sidebar.php.this code will help you.                    


                        global $post;
                        $parent_id  = $post->post_parent; 
                        if(!empty($parent_id)){
                        $parent_post=get_post($parent_id);
                        echo \'<h1 class="entry-title">\'.$parent_post->post_title.\'</h1>\';
                            echo \'<ul>\';
                            $children = wp_list_pages(\'title_li=&child_of=\' . $parent_id . \'&echo=0\');
                            if ($children) {
                            echo $children;
                            echo \'</ul>\';
                            } 
                         } else { 
                        echo \'<ul>\';
                        $page =$post->ID;
                        $children = wp_list_pages(\'title_li=&child_of=\' . $page . \'&echo=0\');
                            if ($children) { 
                            echo $children;
                            } 
                        echo \'</ul>\';
                        }