list pages only from master parent

This is what I have used in a few projects that always displays the children pages even if you’re are viewing a child page. See if this is what you had in mind

<?php
if(!$post->post_parent){
    // will display the subpages of this top level page
    $children = wp_list_pages("title_li=&child_of=".$post->ID."&echo=0");
    $ancestors = $post->ID;
}else{
    // diplays only the subpages of parent level
    //$children = wp_list_pages("title_li=&child_of=".$post->post_parent."&echo=0");
    if($post->ancestors){
        // now you can get the the top ID of this page
        // wp is putting the ids DESC, thats why the top level ID is the last one
        $ancestors = end(get_post_ancestors( $post->ID ));
        $children = wp_list_pages("title_li=&child_of=".$ancestors."&echo=0");
        // you will always get the whole subpages list
    }
}
if ($children) { ?>
    <div class="feature-box">
    <div class="feature-title">Related Links</div>
    <div class="subnav subnav-parent-<?php echo $ancestors; ?>">
        <ul>
            <?php echo $children; ?>
        </ul>
    </div>
</div>
<?php } ?>