Generate navigation based off parent, no matter if you are viewing children or grand children pages.

Sub navigation on children and grand children pages.php

<nav class="sub-menu-wrap">
    <ul class="sub-menu">

    <?php
      $children     = wp_list_pages('depth=1&title_li=&child_of='.$post->ID.'&echo=0');
      $current_page = wp_list_pages('title_li=&include='.$post->ID.'&echo=0');
      $grandparent=0;
      if($post->post_parent):
          $parent          = wp_list_pages('title_li=&include='.$post->post_parent.'&echo=0');
          $siblings        = wp_list_pages('depth=1&title_li=&child_of='.$post->post_parent.'&exclude='.$post->ID.'&echo=0');
          $parent_children = wp_list_pages('depth=1&title_li=&child_of='.$post->post_parent.'&echo=0');
          $parent_post     = get_post($post->post_parent);
          $grandparent     = $parent_post->post_parent;
          $parent_siblings = wp_list_pages('depth=1&title_li=&child_of='.$grandparent.'&echo=0&exclude='.$post->post_parent);
      endif;
      if(!$post->post_parent && $children):?>
        <?php echo $children;?>

    <?php elseif($post->post_parent && $children):?>
        <?php echo $current_page;?>
        <?php echo $children;?>

    <?php elseif($grandparent && $post->post_parent && !$children):?>
        <?php echo $parent;?>
        <?php echo $parent_children;?>

    <?php elseif(!$grandparent && $post->post_parent && !$children):?>
        <?php echo $parent_children;?>
    <?php endif;?>

    </ul>
</div>