WordPress shortcode to list pages based on top-parent of current page (or user-specified parent).

wp_parent_page_family_list_shortcode.php

<?php
/* Shortcode to list page family tree, for current parent.
 *
 * Optional shortcode attributes: parent (ID), a depth, and title.
 * [my_page_family_list parent="123" depth="2" title="My List Title"]
 *
 * @param array $atts Array of settings for wp_list_pages().
 *
 * @return string $list HTML list (nested) of page-family links.
 */
function my_page_family_list_shortcode( $atts ) {
    if ( is_page() ) {
        $atts = shortcode_atts( array(
            'parent' => 0,
            'depth'  => 0,
            'title' => '',
        ), $atts, 'hvpages' );

        $parent = $atts['parent'];
        $depth  = $atts['depth'];
        $title  = $atts['title'];

        // If parent not specified, use page's parent, if any, else use page ID.
        if ( $parent && is_int( $parent ) && get_post_type( $parent ) === 'page' ) {
            $parent_id = $parent;
        } else {
            // Get top-level page ID.
            $page_id    = get_the_id();
            $parent_ids = get_ancestors( $page_id, 'page', 'post_type' );
            $parent_id  = ( $parent_ids ) ? end( $parent_ids ) : $page_id;
            $link_class = ( $page_id == $parent_id ) ? 'current_page_item' : 'page_item';
        }

        $link_text = ( $title ) ? $title : get_the_title( $parent_id );

        $list  = "<nav><h3 class=\"$link_class\">";
        $list .= '<a href="' . get_permalink( $parent_id ) . '"">';
        $list .= $link_text . '</a></h3><ul>';
        $list .= wp_list_pages( array(
                'title_li' => '',
                'child_of' => $parent_id,
                'depth'    => $depth,
                'echo'     => $false,
                'sort_column' => 'menu_order',
        ) );
        $list .= '</ul></nav>';

        return $list;

    } else { // Only for pages.
        return;
    }
}
add_shortcode( 'my_page_family_list', 'my_page_family_list_shortcode' );
?>