Three menus – parent / child / sibling

To list top level pages only:

wp_list_pages( 'depth=1' );

To list children of the current page, but only if it is a top level page:

if ( 0 == $post->post_parent) {
    wp_list_pages( 'child_of=" . $post->ID . "&depth=1' );
}

To list children of the current page, but only if it is a child page:

if ( 0 != $post->post_parent) {
    wp_list_pages( 'child_of=" . $post->ID . "&depth=1' );
}