Plugin to login/register in the sidebar???

You don’t need a plugin. It is a simple call that you can copy/paste anywhere in your sidebar in the theme editor

<!– ADMIN CONSOLE –>
<?php _e(‘Admin’); ?>

<?php wp_register(); ?>

<?php wp_loginout(); ?>

or something like that. When you see a theme you like, look a bit at the code. It’s not hard to copy/paste the order of things in the sidebar. It’s your first step to becoming a hacker.

To add to holly’s reply:

https://codex.www.ads-software.com/Template_Tags/wp_register
https://codex.www.ads-software.com/Template_Tags/wp_loginout

DDT, you can also try inserting the code from here in your sidebar

https://www.homelandstupidity.us/2005/04/14/wordpress-sidebar-login-form-hack/

Though you may need to modify it slightly to make it fit with your theme

Someone else has made a plugin from the code, which you could try instead:
https://template.sfong.net/?p=35