Check whether the child page has siblings

Of course it is possible. Just take a look at docs for wp_list_pages

So something like this will do the trick:

<?php
    global $post; // assuming there is global $post already set in your context
    wp_list_pages( array(
        'exclude' => $post->ID,  // exclude current post
        'parent' => $post->post_parent  // get only children of parent of current post
    ) );
?>

And if you want to use some custom HTML, then something like this should help:

<?php
    global $post; // assuming there is global $post already set in your context
    if ( $post->post_parent ) :  // if it's a child
        $siblings = new WP_Query( array(
            'post_type' => 'page',
            'post_parent' => $post->post_parent,
            'post__not_in' => array( $post->ID )
        ) );
        if ( $siblings->have_posts() ) :
?>
    <ul>
        <?php while ( $siblings->have_posts() ) : $siblings->the_post(); ?>
        <li><a href="https://wordpress.stackexchange.com/questions/304136/<?php the_permalink(); ?>"><?php the_title(); ?></a></li>
        <?php endwhile; wp_reset_postdata(); ?>
    </ul>
<?php 
        endif;
    endif;
?>

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