A shortcode to list the siblings and children of the current page (or any other hierarchical post type)

list-siblings-and-children.php

<?php
/**
 * Add a shortcode to list the siblings and children of the current page.
 * Usage: [list-siblings-and-children]
 *
 * @param array $atts Shortcode attributes.
 * @return string The HTML code to render.
 */
function yoko_list_siblings_and_children( $atts ) {
	global $post;
	$output = '';

	// Parse shortcode attributes.
	$atts = shortcode_atts(
		array(
			'hide_current' => 'false',
		),
		$atts
	);

	// Check if we are on a hierarchical post type.
	if ( is_singular() && is_post_type_hierarchical( get_post_type( $post->ID ) ) ) {
		// Get the parent post ID.
		$parent_id = $post->post_parent;

		// Prepare arguments for getting sibling posts.
		$args          = array(
			'post_type'      => get_post_type( $post->ID ),
			'posts_per_page' => -1,
			'post_parent'    => $parent_id,
			'orderby'        => 'title',
			'order'          => 'ASC',
			'post__not_in'   => 'true' === $atts['hide_current'] ? array( $post->ID ) : array(), // Conditionally exclude current post.
		);
		$sibling_posts = get_posts( $args );

		// Start the list.
		$output .= '<ul class="siblings-and-children">';

		foreach ( $sibling_posts as $sibling ) {
			// Add sibling posts with 'sibling' class.
			$class   = $sibling->ID === $post->ID ? 'current' : 'sibling';
			$output .= '<li class="' . esc_attr( $class ) . '"><a href="' . esc_url( get_permalink( $sibling->ID ) ) . '">' . esc_html( get_the_title( $sibling->ID ) ) . '</a>';

			// Check for child posts.
			$child_args  = array(
				'post_type'      => get_post_type( $post->ID ),
				'posts_per_page' => -1,
				'post_parent'    => $sibling->ID,
				'orderby'        => 'title',
				'order'          => 'ASC',
			);
			$child_posts = get_posts( $child_args );

			if ( ! empty( $child_posts ) ) {
				$output .= '<ul>';
				foreach ( $child_posts as $child ) {
					// Add child posts with 'child' class.
					$class   = $child->ID === $post->ID ? 'current' : 'child';
					$output .= '<li class="' . esc_attr( $class ) . '"><a href="' . esc_url( get_permalink( $child->ID ) ) . '">' . esc_html( get_the_title( $child->ID ) ) . '</a></li>';
				}
				$output .= '</ul>';
			}

			$output .= '</li>';
		}

		$output .= '</ul>';
	}

	return $output;
}
add_shortcode( 'list-siblings-and-children', 'yoko_list_siblings_and_children' );