Add siblings field to WP REST API data

functions.php

<?php

add_action( 'rest_api_init', 'slug_register_siglings' );
function slug_register_siglings() {
	register_api_field( 'post',
		'siblings',
		array(
			'get_callback'    => 'slug_get_siglings',
			'update_callback' => null,
			'schema'          => null,
		)
	);
}

function slug_get_siglings( $object, $field_name, $request ) {
	$next_post = get_adjacent_post( false, '', false );
	$prev_post = get_adjacent_post( false, '', true );

	return array(
		'next'            => $next_post->ID,
		'next_post_title' => $next_post->post_title,
		'prev'            => $prev_post->ID,
		'prev_post_title' => $prev_post->post_title,
	);
}