How To List Sibling Pages And Include The Featured Image?

wp_list_pages only displays page title and link in list. Try following code to fetch page content and thumbnail.

<ul class="pages-list">
           <?php $our_pages = get_pages($args); ?>
           <?php if (!empty($our_pages)): ?>
            <?php foreach ($our_pages as $key => $page_item): ?>
              <li>
                <a href="https://wordpress.stackexchange.com/questions/147745/<?php echo esc_url(get_permalink($page_item->ID)); ?>"><?php echo $page_item->post_title ; ?></a>
                <?php echo get_the_post_thumbnail($page_item->ID,'thumbnail'); ?>
              </li>
            <?php endforeach ?>
           <?php endif ?>
           </ul>