How to only show parent categories in sidebar

Question

You can create a custom widget for that.

// Widget Class ==============================
class WPSE154979_Widget extends WP_Widget
{
    function WPSE154979_Widget()
    {
        $widget_ops = array(
            'classname'     => 'WPSE154979_custom_widget',
            'description'   => __('Post Category Children\'s or Parent Categories')
        );
        $control_ops = array( 'width' => 200, 'height' => 400);
        $this->WP_Widget( 'WPSE154979_custom', 'Custom Categories', $widget_ops, $control_ops );
        $this->alt_option_name="WPSE154979_custom";
    }


    function widget( $args, $instance)
    {
        extract( $args);
        $title = apply_filters( 'widget_title', $instance['title'], $instance, $this->id_base );

        echo $before_widget;
        if( $title )
            echo $before_title . $title . $after_title;

        // on single post page
        if( is_single() && is_object_in_taxonomy( get_post_type(), 'category' ) )
        {
            $cats = wp_get_object_terms( 
                get_the_ID(), 
                'category', 
                array( 'fields' => 'ids', 'orderby' => 'count', 'order' => 'DESC',  ) 
            );
            $parent_id = array_shift( $cats );
            wp_list_categories( 'title_li=&show_option_none=&hide_empty=0&parent=". $parent_id );
        }

        // on category page
        elseif( is_category() )
        {
            $parent_id = (int) get_query_var("cat');
            wp_list_categories( 'title_li=&show_option_none=&hide_empty=0&parent=". $parent_id );
        }

        // on others page
        else
        {
            $parent_id = 0;
            wp_list_categories( "title_li=&show_option_none=&hide_empty=0&parent=". $parent_id );
        }

        echo $after_widget;
    }
    function update( $new_instance, $old_instance ) {
        $instance                   = $old_instance;
        $instance["title']          = strip_tags( $new_instance['title']);
        return $instance;
    }
    function form( $instance )
    {
        $title = isset($instance['title']) ? esc_attr($instance['title']) : ''; ?><p>
            <strong><?php _e( 'Title:' ); ?></strong>
            <br /><input class="widefat" id="<?php echo $this->get_field_id('title'); ?>" name="<?php echo $this->get_field_name('title'); ?>" type="text" 
            value="<?php echo $title; ?>" />
        </p><?php
    }
}


// Register Widget ==============================
add_action('widgets_init', 'WPSE154979_Widget_Init');
function WPSE154979_Widget_Init(){
    register_widget( 'WPSE154979_Widget' );
}

If you add the above code to you themes functions.php file, you would see a new available widget on your WP Admin -> Widgets page. Drag and Drop it to a proper place.

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