Weird child page behavior

Tried to post this in WP-Advanced, but couldn’t so posting it here. I am having the weirdest problem I have ever seen on WP. It’s so weird that I haven’t seen it on any post, and I’ve been googling it for about three hours. I can’t link you because it’s a client site, but I’ll write down the exact setup here.

I have 4 or 5 main pages, all with subpages. One of these main pages is called About (ID is 2), and it has 5 subpages, like so:

• About

• The Designer
• The Services
• NDA
• Recent Work
• Quote

On the sidebar for the pages, I have this code to show either the subpages or the sibling pages:

<?php $children = get_children("post_parent=" . $post->ID );
if( $children )
{
	$output = wp_list_pages('title_li=&child_of=' .$post->ID. '&hierarchical=1&depth=1&echo=0');
	} else {
	$output = wp_list_pages('title_li=&child_of=' .$post->post_parent. '&hierarchical=1&depth=1&echo=0');
				}
				if($output) {
			?>
			<li>
				<ul class="submenu">
					<?php print $output; ?>
				</ul>
			</li>
			<?php }  ?>

Now, it works fantastically on every other page, except for 2 pages that are children of About.

So I thought to myself that there must be a problem with the code somewhere, so I placed a wp_list_pages function with the ID of the About instead of $post->post_parent. It worked great. So, I thought to go back to the root and see if it’s seeing its parent well. I placed this code in one of the pages in which the submenu doesn’t display (page ID is 6):

<?php $parent_title = get_the_title($post->post_parent); echo $parent_title;?>
<?php $parent_id = get_the_id($post->post_parent); echo $parent_id;?>

And, the result for that is : About6.

I went in and checked if it’s a child page in the WP admin, and it is. I’ve resaved it about 10 times, and it still displays the same.

I don’t know what the hell is going on… Help? Please? I don’t know where to look anymore, and I’m pretty sure that the initial code is correct because I’ve used it before on other sites.