The below code is for listing the custom post types itself and its children in the sidebar. The code works great but does not highlight the sub pages.

In functions.php:

if(!function_exists('get_post_top_ancestor_id')){
/**
* Gets the id of the topmost ancestor of the current page. Returns the current
* page's id if there is no parent.
* 
* @uses object $post
* @return int 
*/
function get_post_top_ancestor_id(){
global $post;

if($post->post_parent){
    $ancestors = array_reverse(get_post_ancestors($post->ID));
    return $ancestors[0];
}

return $post->ID;
}}

and adding this code to the sidebar:

<ul>
<?php
$getid=get_post_top_ancestor_id(); 
#echo $getid;
global $post;
$post=get_post($getid);
setup_postdata($post);
?>
<?php
if (is_single($post->ID)) {$pg_li .="current_page_item";}
else {
$pg_li .="page_item";}
?>
<h3><?php the_title();?></h3>
<p></p>
<li class="<?php echo $pg_li; ?>">
<a href="<?php the_permalink(); ?>">BIOGRAPHY</a>
</li>
<li class="<?php echo $pg_li; ?>">
<?php
$args=array(
'post_type'=>'artists',
'child_of'=>$getid ,
'sort_column'=>'menu_order',
'hierarchical' => 1,
'title_li'=>__('')
);
wp_list_pages( $args );
wp_reset_query();
?></li>
</ul>

I am not familiar with PHP, and a friend helped me to write this code. So what am I missing to highlight the current sub menu of that custom post type?