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
if($post->post_parent) {
$children = wp_list_pages("title_li=&child_of=".$post->post_parent."&echo=0$sort_column=menu_order");
$title_heading = get_the_title($post->post_parent);
}
else {
$children = wp_list_pages("title_li=&child_of=".$post->ID."&echo=0");
$title_heading = get_the_title($post->ID);
}
if ($children) { ?>
<h2> <? echo $title_heading ?> </h2>
<ul>
<?php echo $children; ?>
</ul>
<?php } ?>