Hi Louisa,
Thank you for writing in, sorry but we can not really recommend a plugin (unless we personally use or fully tested it). My suggestion is instead of redirecting your login page, create a custom WordPress Login page instead, please see the following article.
How to Create a Custom WordPress Login Page
But please be reminded that this is customization and we can not provide support for it.
You might also want to take a look at the White Label Extension if you only want to change the Wordpress logo and add background-image on the default login page.
Cheers!