How to handle many different menus?

If I understand your question right, here is the code to show the list of page siblings (having the same parent). Call the function from the sidebar.

<?php
function show_page_siblings( $id ) {

    $parent_id = wp_get_post_parent_id( $id );

    // show only on child pages
    if( 0 == $parent_id ) {
        return;
    }

    $siblings = get_pages( array(
        'child_of'     => $parent_id,
        'hierarchical' => 0,
        'parent'       => $parent_id,
        'post_type'    => 'page',
        'post_status'  => 'publish'
    ));

    if( !empty( $siblings ) ) {
        echo '<ul>';

        foreach( $siblings as $sibling ) {
            echo '<li><a href="' . get_permalink( $sibling->ID ) . '">' . $sibling->post_title . '</a></li>';
        }

        echo '</ul>';
    }

}