Get siblings of post. Remove . '&exclude=' . $post->ID to include current post.

post_siblings.php

<?php 
// global $post; //not necessary if used in the loop
  $parent_id = $post->post_parent;
  if( $parent_id ) :
  $siblings = get_pages( 'child_of=' . $parent_id . '&parent=' . $parent_id . '&exclude=' . $post->ID);
    if( $siblings ) foreach( $siblings as $sibling ) :
    //start of whatever you need to output//
      echo $sibling->post_title;
      echo get_the_post_thumbnail($sibling->ID,'medium');
    //end of whatever you need to output//
    endforeach;
  endif; //ends if( $parent ) //
?>  

// Includes current post and active class
<nav class="subnav">
                <?php 
                      $parent_id = $post->post_parent;
                      if( $parent_id ) :
                      $siblings = get_pages( 'child_of=' . $parent_id . '&parent=' . $parent_id);
                        if( $siblings ) : 
                            echo "<ul>";
                            foreach( $siblings as $sibling ) :
                                if($sibling->ID == $post->ID) :
                                    echo '<li class="active"><a>'.$sibling->post_title.'</a></li>';
                                else :
                                    echo '<li><a>'.$sibling->post_title.'</a></li>';
                                endif;
                            endforeach;
                            echo "</ul>";
                        endif; //ends if( $parent ) //
                      endif; //ends if( $parent ) //
                ?>  
            </nav>