List all the children of parent page

When you visit the parent page, it will list all its children. Visiting the child page of the parent will show its siblings.

<?php if($post->post_parent)
   $children = wp_list_pages("sort_column=menu_order&title_li=&depth=1&child_of=".$post->post_parent."&echo=0");
   else $children = wp_list_pages("sort_column=menu_order&title_li=&depth=1&child_of=".$post->ID."&echo=0");
   if ($children): ?>
   <ul>
     <?php echo $children; ?>
   </ul>
<?php endif; ?>