Use the following to change the query from querying the current page to querying the parent of the current page:

query_posts( 'p='.$post->post_parent  );

That will change the $wp_query object to hold only the parent of the current post.

Note: this is untested.

update

You can also do it with a custom WP_Query as so:

$args = array(
    'p' => $post->post_parent
);
$parent = new WP_Query( $args );

while( $parent->have_posts() ) : $parent->the_post();
    // loop stuff here
endwhile;