Navigation problem – showing child pages in navigation

I’m having an issue with showing child page navigation. When I’m viewing the parent of children pages, I can see the child pages listed, but when I go to a child pages, their siblings disappear. Here’s the code (I’m using the ‘Round Flow’ template):

<div id="navigation">
<ul>
<!-- <li><a href="<?php bloginfo('home'); ?>">Home</a></li> -->
<?php wp_list_pages('sort_column=menu_order&title_li=&depth=1'); ?></ul>
</div>

<?php if(is_page() && wp_list_pages("child_of=".$post->ID."&echo=0")) { ?>
<div id="childnavigation">
<ul>
<?php wp_list_pages("title_li=&child_of=".$post->ID."&sort_column=menu_order");?>
</ul>
</div>

I’m not a pro, but I believe what I want to do is say something like, “if child of a page, show that parent pages children pages in the navigation”- but I’m not sure how to do this.

I would be willing to switch the navigation to a drop-down format that would be able to show all the children on any pages, but I can’t find a template that has that for the pages. (I’m using WP to build a whole website, and not currently using the blog posts, which is why that line is commented out in the code above).

Thank you.