2 comments

1. This is what I have used in a few projects that always displays the children pages even if you’re are viewing a child page. See if this is what you had in mind

   <?php
   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");
       $ancestors = $post->ID;
   }else{
       // diplays only the subpages of parent level
       //$children = wp_list_pages("title_li=&child_of=".$post->post_parent."&echo=0");
       if($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(get_post_ancestors( $post->ID ));
           $children = wp_list_pages("title_li=&child_of=".$ancestors."&echo=0");
           // you will always get the whole subpages list
       }
   }
   if ($children) { ?>
       <div class="feature-box">
       <div class="feature-title">Related Links</div>
       <div class="subnav subnav-parent-<?php echo $ancestors; ?>">
           <ul>
               <?php echo $children; ?>
           </ul>
       </div>
   </div>
   <?php } ?>
   

2. Your current logic is:

   if we are a root page: ( we're 1 level deep! )
       show the children!
   but if we are not a root page: ( we're 2 levels deep! )
       show the siblings/parents children!
   

   And your issue is what happens when you arrive on a page that’s 3 levels deep? We can check if we have a parent or not, but we also need to check if that parent has a parent too, aka a grandparent.

   So this would do the trick:

   if we are a root page: ( we're 1 level deep! )
       show the children!
   but if we are not a root page: ( we're 2 or more levels deep! )
       get the parent post
       does the parent post have a parent too? AKA  a grandparent ( we're 3 levels deep! )
           show the grand parents children!
       but if the parent post does not have a parent ( we're 2 levels deep! )
           show the parents children!
   

   But clearly this doesn’t scale! What about 4 levels deep? 5 levels deep? What if my page has a parent, which has a parent, which has another parent? Too many levels! So confusing, and so many nested if else statements.

   So how do we do this correctly?

   Recursion

   For a truly robust answer, we need to use a recursive function. We don’t know how many levels deep we are, so we can’t account for every single level. Not without having huge if else statements and nightmarish code.

   So instead, lets do something similar to this:

   For post A
   do the "show posts" routine to A
   

   Where “show posts” is:

   show posts:
       if A has no parent
           Display the children!!
       but if A DOES has a parent
           do the "show posts" routine on that parent
   

   In more ‘PHP-like’ code: note, this is pseudocode, not actual code

   function showPosts( $post ) {
       if ( $post->post_parent == 0 ) {
           // display the child posts
       } else {
           // go up one level and try again
           showPosts( $post->post_parent );
       }
   }
   $this_page = $wp_query->post;
   showPosts( $this_page );
   

   Now we have something that will work for 1 level deep, 2 levels deep, 3 levels deep ….. 5000 levels deep, 5001 levels deep, etc etc etc

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