Next and Prev Page Siblings - Wordpress

next-prev-sibling.php

<?php

function getId($page) { return $page->ID; }  

$ancestors = get_post_ancestors( $post->ID );
$parent = ($ancestors) ? $ancestors[0] : null;

if ($parent) :
   $siblingIds = array_map( "getId", get_pages('child_of='. $parent .'&sort_column=menu_order&sort_order=asc') );

   $current = array_search( get_the_ID(), $siblingIds );
   $prevId = ( isset($siblingIds[$current-1]) ) ? $siblingIds[$current-1] : '';
   $nextId = ( isset($siblingIds[$current+1]) ) ? $siblingIds[$current+1] : '';

?>

<nav id="pagination">
    <?php if (!empty($prevId)) : ?>
      <div class="alignleft">
         <a 
           href="<?php  echo get_permalink($prevId); ?>"
           title="<?php  echo get_the_title($prevId); ?>" 
           class="previous-page"
         >&lt;&nbsp;&nbsp;Previous</a>
    </div>
    <?php endif; ?>
   
    <?php if (!empty($nextId)) : ?>
       <div class="alignright">
          <a
             href="<?php echo get_permalink($nextId); ?>"
             title="<?php  echo get_the_title($nextId); ?>"
             class="next-page"
          >Next&nbsp;&nbsp;&gt;</a>
       </div>
    <?php endif; ?>
</nav>

<?php endif; ?>

Changed the old $pages variables in lines 12 and 13 by the actual $siblingIds variable