How to get products list from the same groups

Version WP: 4.8.1
Version Woo: 3.1.2

In simple product page I want to show products list from the same groups.

Example:
Group1(grouped product)
– product1
– product2
– product3

Group2(grouped product)
– product1
– product4
– product5

In product page “product1” I want to show list products consist of product2, product3, product4, product5

In product page “product2” I want to show list products consist of product1 and product3

Before Woocommerce 3.0.0 I used this function in file content-single-product.php:

if( $post->post_parent != 0 || $product->is_type( 'grouped' ) ) {

	if( $post->post_parent != 0 ) {
		$parent_id = $post->post_parent;
	} else {
		$parent_id = $post->ID;
	}
	$parent_product = get_product( $parent_id );
	$childrens = $parent_product->get_children();
	if (!empty($childrens)) {

		global $product, $woocommerce_loop;

		$args = array(
			'post_type'				=> 'product',
			'ignore_sticky_posts'	=> 1,
			//'no_found_rows' 		=> 1,
			'posts_per_page' 		=> -1,
			'orderby' 				=> $orderby,
			'post__in' 				=> $childrens
			);

		$parent_products = new WP_Query( $args );

		$woocommerce_loop['columns'] 	= $columns;

		if ( $parent_products->have_posts() ) { ?>

			<div class="parent-products products compact-layout grid--float">
				<div class="title">
					<h4><?php printf( __( 'Collection Products &ldquo;%s&rdquo;', 'mk_framework' ), get_the_title($parent_id) ); ?></h4>
					<img alt="" title="" width="150" height="24" src="<?php echo get_site_url(); ?>/wp-content/uploads/2016/12/div.png">
				</div>
				<section class=" mk--row">
					<?php woocommerce_product_loop_start(); ?>

					<?php while ( $parent_products->have_posts() ) : $parent_products->the_post(); ?>

					<?php wc_get_template_part( 'content', 'product' ); ?>

					<?php endwhile; // end of the loop. ?>

					<?php woocommerce_product_loop_end(); ?>
				</section>
			</div>
		<?php }
	}
}

But $post->post_parent not working, and return 0. What alternative? How do I get a list ID’s grouped products for sample product?

Thank!

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