Plugin to show pagemap beneath certain page and next/previous page beneath certain page?

This will get you the parent page ID:

$post->post_parent

..and this will get that parent page’s children:

get_children($post->post_parent);

By default that will return an object. You can then play about with that object to your heart’s content, including making a navigation of sorts.

Something like this to get started:

<ul>
    <li class="heading"><?php get_title($post->post_parent); ?></li>
    <?php

        $children = get_children($post->post_parent);

        foreach($children as $c) {
            echo '<li>'.get_title($c->ID).'</li>';
        };

    ?>
</ul>