Include parent page in list of child pages

You can get the parent with wp_get_post_parent_id(get_the_ID()), and store it in a new variable and do the same with the children and then just showcase them in any format you want.

<?php
$theParent = wp_get_post_parent_id(get_the_ID());

if ( $theParent->ID == 83 ) { ?>
  <div class="parent"><?PHP echo get_the_title($theParent); ?></div>
  <ul class="children"><?PHP wp_list_pages(array(
            'title_li' => '',
            'child_of' => $theParent ?> </ul>
 <?PHP } ?>

Hopefully, this helps