List children on child post

You can get the list of other posts with same post parent by running a custom query. You should supply parent post id in post_parent parameter to WP_Query and exclude current post from custom query.

So this will be the custom query.

$parentpost = wp_get_post_parent_id( $post->ID );

if ( $parentpost && $parentpost != '0' ) {

    $currentpost = array( $post->ID );

    $args = array(
        'post_type' => 'cpt-name',
        'post_parent' => $parentpost,
        'post__not_in' => $currentpost,
        'ignore_sticky_posts' => 1,
        'paged' => $paged,
    );

    $my_query = new WP_Query( $args );

    if ( $my_query->have_posts() ) :

        while ( $my_query->have_posts() ) : $my_query->the_post();
            the_title();
        endwhile;

    endif;

    wp_reset_postdata();

} else {

    echo "Do something else.";

}

Oh and also, you will have to add your custom post type name in above code to work. I added cpt-name right now as an example. I hope this helps.