Show child pages when on a child page

On my site I have a number of parent pages with associated child pages. How can I show all the child pages of one particular parent when a visitor is either on the parent page or one of it’s children?

For example;

If someone clicks onto the “Story” parent page, they’ll see a list of “story” child pages in the sidebar (As well as a link to the parent page). Then if they click through to one of those child pages, they’ll still be able to see the list in the sidebar

    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>';
                        }