Using query_posts to return a page’s immediate sibling

Basically, what I want to do is create a next_post_link, but for WP Pages. Let’s say this is my page hierarchy:

Page A
Page B
Page C

While I’m viewing Page A, I want to display a link and a preview image of Page B. While viewing Page B, I want to view the same for Page C, and so on down the line.

Here’s my code so far:

<?php
global $post;
$currentpage = get_the_ID();
$parent = $post->post_parent;
$args=array(
'post_type'=>'page',						'post__not_in'=>array($currentpage),				'post_parent' => $parent,
'orderby' => 'title',
'order' => 'ASC',
'showposts' => 1,
);

$siblings = new WP_Query($args);
?>
<div class="next-page">
<?php while ($siblings->have_posts()) : $siblings->the_post(); ?>
<!--do some stuff-->
<?php endwhile ;?>

What I’m missing is some sort of offset, or “start” parameter, so that it only return the immediate sibling of the current page.

I appreciate any help. Thanks.