If the page is a parent then show child page links, if the page does not page child pages, show pages links of parent.

WordPress - show page link is page is parent or child

    // https://wordpress.stackexchange.com/questions/35724/function-to-return-true-if-current-page-has-child-pages
    function wpb_list_child_pages() { 
     
        global $post; 

        // does page have children?
        $children = get_pages( array( 'child_of' => $post->ID ) );
        if( count( $children ) == 0 ) {
            // not a parent page,
            // show children page links of parent
            $childpages = wp_list_pages( 'depth=1&sort_column=menu_order&title_li=&child_of=' . $post->post_parent . '&echo=0' );

        } else {
            // is a parent page
            // show child page of this page
            $childpages = wp_list_pages( 'depth=1&sort_column=menu_order&title_li=&child_of=' . $post->ID . '&echo=0' );
        }

         
        if ( $childpages ) {
         
            $string = '<ul>' . $childpages . '</ul>';
        }
         
        return $string;
     
    }
    add_shortcode('wpb_childpages', 'wpb_list_child_pages');