send_email_to_admins.php

<?php
/**
 * This recipe will forward the Membership Expiring email to another email address
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function my_pmpro_email_recipient($headers, $email) {
    
    if(strpos($email->template, 'membership_expiring')){
		$headers[] = "Bcc:" . "otheremail@domain.com";		
}
    return $headers;
}
add_filter('pmpro_email_recipient', 'my_pmpro_email_recipient', 10, 2);