Wordpress page menu that shows all top-level pages, and child pages when EITHER the current page has children, OR the current child page has siblings.

Custom WP page menu

<?php
$all_pages = get_pages();

foreach ($all_pages as $page) {
	if (!$page->post_parent) {
		$incl .= $page->ID . ",";
	} elseif ($page->post_parent == $post->ID ||
				$page->post_parent == $post->post_parent) {
		$incl .= $page->ID . ",";
	} else {
		continue;
	}
}
?>

<ul>
<?php 
wp_list_pages(array(
	"include" => $incl, 
	"link_before" => "", 
	"title_li" => "", 
	"sort_column" => "menu_order"
));
?>
</ul>