Auto expanding navigation (Only siblings of parents displayed)

gistfile1.twig

{% set topEntry = craft.entries.depth(1).slug('about-us').first() %}
{% set parents = entry.getAncestors() %}

<ul>
	{% nav item in topEntry.getDescendants() %}

		{% set showChildren = false %}

		{% for parentItems in parents %}
			{% if parentItems.id == item.id or entry.id == item.id %} 
				{% set showChildren = true %} 
			{% endif %}
		{% endfor %}

		<li class="level_{{ item.depth }}">{{ item.link }} 

		{# fine #}
		{% if showChildren %}
			x
		{% endif %}

		{# shits itself here #}
		{% if showChildren %} 
			{% ifchildren %}
			<ul>{% children %}</ul>
			{% endifchildren %}
		{% endif %}

		</li>

	{% endnav %}
</ul>