Show child pages of parent, or if viewing child page, it will get all pages associated with parent.

Wordpress child pages of parent.php

<aside>

  <nav class="page-sub-navigation">

    <?php
      // get child pages from post parent, otherwise
      // get child pages from the current page being viewed, in most cases the parent
      if ($post->post_parent) {
        $page_list = wp_list_pages( 'title_li=&display=0&child_of='.$post->post_parent.'&echo=0' );
      } else {
        $page_list = wp_list_pages( 'title_li=&display=0&child_of='.$post->ID.'&echo=0' );
      }
      // if child pages exist, load them in otherwise
      if ($page_list):
        echo '<ul>';
          echo $page_list;
        echo '</ul>';
      endif;
    ?>

  </nav>

</aside>