Puxar as páginas filha da página atual. Mas se não tiver filhas, puxar as irmãs.

page.php

<?php

  $args = array(
      'post_type'      => 'page',
      'posts_per_page' => -1,
      //'post_parent'    => $post->ID,
      'post_parent'    => $post->post_parent,
      'order'          => 'ASC',
      'orderby'        => 'title'
  );

  $child = array(
    'post_type'      => 'page',
    'posts_per_page' => -1,
    'post_parent'    => $post->ID,
    //'post_parent'    => $post->post_parent,
    'order'          => 'ASC',
    'orderby'        => 'title'
);


  $parent = new WP_Query( $args );
  $childs = new WP_Query( $child );
  if ( $parent->have_posts() ) : 
?>
    <ul id="parent-<?php the_ID(); ?>" class="parent-page section-menu-page">
        <?php while ( $parent->have_posts() ) : $parent->the_post(); ?>
            <li><a href="<?php the_permalink(); ?>" title="<?php the_title(); ?>"><?php the_title(); ?></a></li>
        <?php endwhile; ?>
    </ul>
<?php if ( $childs->have_posts() ) :?>
teste
<?php endif; wp_reset_postdata(); ?>