Following code will display all the subpages of same parent page. also displays the subpages of current page if it is not a subpage.

<?php
// Get Parent of current page....
$parents_id = $wpdb->get_row("SELECT post_parent	FROM $wpdb->posts WHERE id = ".$post->ID."	AND post_type = 'page' ORDER BY menu_order", 'OBJECT');	

if($parents_id->post_parent >0) // If this page have parent
{
	// Get all the subpages of the parent page.
	$child_pages = $wpdb->get_results("SELECT *	FROM $wpdb->posts WHERE post_parent = ".$parents_id->post_parent."	AND post_type = 'page' ORDER BY menu_order", 'OBJECT');
if(count($child_pages))// If count is greater than 0
{
?>
<ul>
<li><strong><?php echo get_the_title( $parents_id->post_parent);?></strong></li>
<?php if ( $child_pages ) : foreach ( $child_pages as $pageChild ) : setup_postdata( $pageChild ); ?>
<li><a href="<?php echo $pageChild->guid; ?>" rel="bookmark" title="<?php echo $pageChild->post_title; ?>"><?php echo $pageChild->post_title; ?></a></li>
<?php endforeach; endif;?>
</ul>
<?php
}
}
else
{
	//If the current page is a top level page and it is not a subpage.
	//Get the subpages of the current page
	$child_pages = $wpdb->get_results("SELECT *	FROM $wpdb->posts WHERE post_parent = ".$post->ID."	AND post_type = 'page' ORDER BY menu_order", 'OBJECT');
	if(count($child_pages))
{
?>
<ul>
<li><strong><?php echo get_the_title( $post->ID);?></strong></li>
<?php if ( $child_pages ) : foreach ( $child_pages as $pageChild ) : setup_postdata( $pageChild ); ?>
<li><a href="<?php echo $pageChild->guid; ?>" rel="bookmark" title="<?php echo $pageChild->post_title; ?>"><?php echo $pageChild->post_title; ?></a></li>
<?php endforeach; endif;?>
</ul>
<?php
}
}
?>