You can use this WordPress plugin to force users to the "/customer-sign-in" slug rather than "/wp-login" during login. Customers that attempt to point their browser to "/wp-login" will be redirected.
To use, simply copy this entire directory into your WordPress plugins directory and then activate the plugin from the WordPress admin panel.
You may also be interested in the `Inesonic WordPress Login Blocker plugin <https://github.com/tuxidriver/inesonic-wp-login-blocker>. Both plugins are used by Inesonic <https://inesonic.com> infrastructure.