Determine if post type is a child of another or is the parent.

is-child-parent-or-sibling-page.php

<?php
/**
 * Check is post is the child, sibling or the parent.
 *
 * Modified from http://css-tricks.com/snippets/wordpress/if-page-is-parent-or-child/
 **/

function is_branch( $pid ) { //  Check if parent or child
	global $post;
	if( is_page()  && is_int( $pid ) && ( $post->post_parent == $pid || is_page( $pid ) ) ) {
		return true;
	} else {
		return false;
	}
}

function is_child ( $pid ) { // Check if child only
	global $post;
	if( is_page() && is_int( $pid ) && ( $post->post_parent == $pid ) ) {
		return true;
	} else {
		return false;
	}
	
}

function is_sibling( $pid ) { // Check if page is a sibling of another
	global $post;
	
	if( is_page()  && is_int( $pid ) && ( $post->post_parent != $pid ) ) {
		$my_ancestors = get_post_ancestors( $post );
		$pid_ancestors = get_post_ancestors( $pid );
		if( !empty( $my_ancestors ) && !empty( $pid_ancestors ) ) {
			if( in_array( $my_ancestors, $pid_ancestors ) )
				return true;
		}
	} else {
		return false;
	}
}