PHP: WordPress | Get children pages AND the sibling pages ONLY when on child pages

gistfile1

<?php
// Get children pages AND the sibling pages ONLY when on child pages 
  if($post->post_parent)
  $children = wp_list_pages("title_li=&child_of=".$post->post_parent."&echo=0&sort_column=menu_order&depth=1");
  else
  $children = wp_list_pages("title_li=&child_of=".$post->ID."&echo=0&sort_column=menu_order&depth=1");
  if ($children) : 
?>
<div id="child-menu" >
	<ul>
		<?php echo $children; ?>
	</ul> 
</div>
<?php endif; ?>