This script will look at the current page and serve up a list of sibling pages.

wordpress-secondary-navigation-script.php

<?php
// Show child or sibling pages in left column.
if(!$post->post_parent){
	// will display the subpages of this top level page
	$children = wp_list_pages("title_li=&child_of=".$post->ID."&echo=0&sort_column=menu_order");
}
else{

	if($post->ancestors) {
		// now you can get the the top ID of this page
		// wp is putting the ids DESC, thats why the top level ID is the last one
		$ancestors = end($post->ancestors);
		$children = wp_list_pages("title_li=&child_of=".$ancestors."&echo=0&sort_column=menu_order");
	}
}

if ($children) { ?>

<div class="sidebarnav">
<p><strong>Inside This Section:</strong></p>
<div class="textwidget">
  <ul class="sidebar-menu-choices">
		<?php echo $children; ?>
	</ul>
</div>
</div>
<?php } ?>