Example of how to redirect the user away from the standard EE login form when the wpuser_login step has been initialized.

example.php

<?php //Please do not include the opening PHP tag if you already have one 

function tw_redirect_login( EE_SPCO_Reg_Step_WP_User_Login $spco_step ) {
    wp_safe_redirect( home_url() );
}
add_action('AHEE__Single_Page_Checkout___initialize_reg_step__wpuser_login', 'tw_redirect_login');