Show posts assigned to multiple categories in current category page

That’s because that’s what you’re asking for:

'category__in' => array($category),

It’s only going to show posts in that category.

You shouldn’t be using get_posts() or WP_Query() in category.php. WordPress has already queried the correct posts. You should only be using the standard loop to display them:

<?php
if ( have_posts() ) :
    while ( have_posts() ) : the_post();
        // Display post content
    endwhile;
endif;
?>