Check if the child page has sibling pages, and bookmark current page

Yes, it is possible:

<?php
    global $post;
    $current_post = $post;
    if ( $post->post_parent ) {  
        $siblings = new WP_Query( array(
            'post_type' => 'page',
            'order' => 'ASC',
            'post_parent' => $post->post_parent
        ) );
    }

    if ( $siblings->have_posts() ) :
?>
    <ul>
        <?php while ( $siblings->have_posts() ) : $siblings->the_post(); ?>
        <li><a href="https://wordpress.stackexchange.com/questions/306078/<?php the_permalink(); ?>">
            <?php if ( $current_post->ID == $post->ID ) echo 'Actual page: '; ?>
            <?php the_title(); ?>
        </a></li>
        <?php endwhile; wp_reset_postdata(); ?>
    </ul>
<?php endif; ?>