Not the cleanest solution, but I use something like this for a few instances of current page menu highlighting for pages that aren’t child/parent related in any way:
<?php if (is_page('tutorials-by-topic') { ?>
<script type="text/javascript">
jQuery(function($) {
$(document).ready(function() {
$('#menu-main-menu li.menu-item-1319').addClass('current-menu-item');
}); });
</script>
<?php } ?>
You have to look at the source to get the correct menu-item-xxxx for the page that you want to highlight as current in the menu.