Getting a value from a custom field from a page that shares a parent with the current page

I’m assuming there are only two children in such cases.

<?php
$child_id = $post->ID;
if ( $post->post_parent ) {
    $args = array(
                    'post_parent' => $post->post_parent,
                    'post_type'   => 'page', 
                    'numberposts' => -1,
                    'post_status' => 'publish' 
                  );
    $children = get_children($args);
    if(count($children) == 2) {
        $child_id = $children[0]->ID == $post->ID ? $children[1]->ID : $children[0]->ID;
    }
} 

if ( get_field( 'show_video_menu' , $child_id ) ): ?>

<li><a id="artists-video" href="https://wordpress.stackexchange.com/<?php global $post;
if($post->post_parent) { $post_data = get_post($post->post_parent);
echo $post_data->post_name; }?>/video">Video</a></li>

<?php endif; ?>