Renders buttons that link to a page's closest siblings

sibling-page-links.php

<?php
/**
 * Calls the page link functions on pages that are descendants of $greatest_ancestor
 *
 * @param int $greatest_ancestor    The ID of the top level ancestor
 */
function tj_page_navigation_links( $greatest_ancestor ) {
    //* Get the ancestors of the current page
    $page_ancestors = get_post_ancestors( get_the_ID() );

    //* Return early if not a descendant of $page_ancestors
    if ( ! in_array( $greatest_ancestor, $page_ancestors) )
        return;

    $args = array(
        'post_parent'	=> $page_ancestors[0],
        'post_status'	=> 'publish',
        'orderby'		=> 'menu_order',
        'order'			=> 'ASC',
    );
    $siblings = array_keys( get_children( $args ) );
    $position = array_search( get_the_ID(), $siblings );

    tj_prev_sibling_link( $siblings, $position );
    tj_next_sibling_link( $siblings, $position );
}

/**
 * Renders the Prev: Page button
 *
 * @param array $siblings   All children of $greatest_ancestor
 * @param int $position     Position of the current page among its siblings
 */
function tj_prev_sibling_link( $siblings, $position ) {
    if ( get_the_ID() == $siblings[0] )
        return;

    //* Render a button that links to the previous sibling page.
    $prev_sibling_permalink = get_the_permalink( $siblings[$position-1] );
    $prev_sibling_title = get_the_title( $siblings[$position-1] );
    printf( '<a class="%s" href="%s">%s</a>', 'button prev', $prev_sibling_permalink, 'Prev: ' . $prev_sibling_title );
}

/**
 * Renders the Next: Page button
 *
 * @param array $siblings   All children of $greatest_ancestor
 * @param int $position     Position of the current page among its siblings
 */
function tj_next_sibling_link( $siblings, $position ) {
    if ( get_the_ID() == end( $siblings) )
        return;

    //* Render a button that links to the next sibling page.
    $next_sibling_permalink = get_the_permalink( $siblings[$position+1] );
    $next_sibling_title = get_the_title( $siblings[$position+1] );
    printf( '<a class="%s" href="%s">%s</a>', 'button next', $next_sibling_permalink, 'Next: ' . $next_sibling_title );
}