acf.php

<?php

/**
 * ACF Rule Type: Page Ancestor
 *
 * @author Bill Erickson
 * @see http://www.billerickson.net/acf-custom-location-rules
 *
 * @param array $choices, all of the available rule types
 * @return array
 */
function ea_acf_rule_type_page_ancestor( $choices ) {
	$choices['Page']['page_ancestor'] = 'Page Ancestor';
	return $choices;
}
add_filter( 'acf/location/rule_types', 'ea_acf_rule_type_page_ancestor' );