WORDPRESS - Show children of page if page is parent, and siblings if page is not a parent

gistfile1.txt

<?php
  						global $post;     // if outside the loop
							if ( is_page() && $post->post_parent ) {
								// This is a subpage
								$mypages = get_pages( array( 'child_of' => $post->post_parent, 'sort_column' => 'post_date', 'sort_order' => 'asc', 'hierarchical' => '0', 'parent' => $post->post_parent ) );
								
								echo '<ul id="nav">';
								
								
								foreach( $mypages as $page ) {		
									if($page->ID == $post->ID)
										echo '<li><a href="'.$page->guid.'" class="active">'.$page->post_title.'</a></li>';		
									else
										echo '<li><a href="'.$page->guid.'">'.$page->post_title.'</a></li>';
								}
								echo '</ul>';

							} else {
								// This is not a subpage
								$mypages = get_pages( array( 'child_of' => $page_id, 'sort_column' => 'post_date', 'sort_order' => 'asc', 'hierarchical' => '0', 'parent' => $page_id ) );
								echo '<ul id="nav">';
								
								foreach( $mypages as $page ) {		
									echo '<li><a href="'.$page->guid.'">'.$page->post_title.'</a></li>';	
								}
								echo '</ul>';
							}
							
								
						?>