previous/next_post_link in the same sub-category?

If you’ll take a look at next_post_link Codex page, you’ll see it’s usage:

<?php
next_post_link(
    $format,
    $link,
    $in_same_term   = false,
    $excluded_terms="",
    $taxonomy       = 'category'
);

As you can see, third param is in_same_term and it’s default value is false. If you set it to true, then the function will return next post from same category. (If you want the term from different taxonomy, you can change fifth param).

So answering your question… Yes, there is a way to achieve this. It can look like so:

<?php next_post_link( '%link &raquo;', '%title', true ); ?>