blog/_src/_includes/relatedposts.html

{% if site.related_posts.size > 1 %}
    <aside class="related-posts">

        <h1 class="related-posts-title">Related</h1>

        <div class="row">

            {% for post in site.related_posts %}

                {% assign match = false %}
                {% for category in post.categories %}
                    {% if page.categories contains category %}
                        {% assign match = true %}
                    {% endif %}
                {% endfor %}

                {% if match %}
                    <article class="related-post col2">
                        <a class="post-title-link" href="{{ post.url }}">
                            <h1 class="post-title">{{ post.title }}</h1>
                        </a>
                    </article>
                {% endif %}

            {% endfor %}

        </div>

    </aside>
{% endif %}