if (isset($parts[0])) {
$this->data['category_id'] = $parts[0];
} else {
$this->data['category_id'] = 0;
}

if (isset($parts[1])) {
$this->data['child_id'] = $parts[1];
} else {
$this->data['child_id'] = 0;
}
//ADDED FOR 2ND LEVEL
if (isset($parts[2])) {
$this->data['sister_id'] = $parts[2];
} else {
$this->data['sister_id'] = 0;
}

//DEFINE 2ND LEVEL
$sisters_data = array();

$sisters = $this->model_catalog_category->getCategories($child['category_id']);

foreach ($sisters as $sister) {
$data = array(
    'filter_category_id'  => $sister['category_id'],
    'filter_sub_category' => true
);

$sisters_data[] = array(
    'category_id' => $sister['category_id'],
    'name'        => $sister['name'],
    'href'        => $this->url->link('product/category', 'path=' .
$category['category_id'] . '_' . $child['category_id'] . '_' . $sister['category_id'])
);
}

<?php if ($category['children']) { ?>
    <ul>
    <?php foreach ($category['children'] as $child) { ?>
    <li>
    <?php if ($child['category_id'] == $child_id) { ?>
        <a href="<?php echo $child['href']; ?>" class="active"> - <?php echo $child['name']; ?></a>
        <?php } else { ?>
        <a href="<?php echo $child['href']; ?>"> - <?php echo $child['name']; ?></a>
        <?php } ?>

    <?php if ($category['sisters']) { ?>
    <ul>
    <?php foreach ($category['sisters'] as $sister) { ?>
        <li>
        <?php if ($sister['category_id'] == $sister_id) { ?>
        <a href="<?php echo $sister['href']; ?>" class="active">  <?php echo $sister['name']; ?></a>
        <?php } else { ?>
        <a href="<?php echo $sister['href']; ?>">  <?php echo $sister['name']; ?></a>
        <?php } ?>
        </li>
    <?php } ?>
    </ul>
    <?php } ?>
        </li>
<?php } ?>
</ul>
<?php } ?>