Body classes in child theme

Thank you for all the insets with this question. Here is what I come up with working from 1fixdotio’s answer above. I first had to unset the full-width body class and then register a new body class to incorporate my new sidebar. Here is the code

  // Remove body class and register new body class to accomodate the new custom-sidebar-per-page sidebars
function remove_a_body_class($classes) {
      foreach($classes as $key => $value) {
      if ($value == 'full-width') unset($classes[$key]);
      }
return $classes;
}

add_filter('body_class', 'remove_a_body_class', 20, 2);

function pietergoosen_add_new_body_class( $classes ) {
$options = get_post_custom($post->ID);  
$sidebar_choice = $options['custom_sidebar_per_page'][0];  
    if ( (! is_active_sidebar( 'sidebar-2' ) && ! is_active_sidebar( $sidebar_choice ) )
        || is_page_template( 'page-templates/full-width.php' )
        || is_page_template( 'page-templates/contributors.php' )
        || is_attachment() ) {
        $classes[] = 'custom-sidebar-per-page-width';
    }

    return $classes;
}
add_filter( 'body_class', 'pietergoosen_add_new_body_class' );