Parents attending thesis defense?

Open to the public means ... open to the public (presuming they can sit quietly in a room for 2 hours or however long your wife's defense is).

Moreover, (edit though I thought I was clear), it's very common in the US for family including parents and spouses to attend PhD defenses.

My wife and parents came to my defense (philosophy PhD USA). As did a friend of ours who was a foreigner living in America with no connection to philosophy. A philosophy PhD defense at least at my institution is them drilling you with every objection and reason that they think you're wrong for two solid hours.

While I assume a nutrition science PhD has a different format, I hope that neither the committee nor the person defending the dissertation has much time to look around and see who came and to think odd thoughts about the professionalism of the candidate based on who came to their defense.

Your wife should ask this question to people in her department.

The style and expectations of defenses vary hugely from one place to another. As other answerers mention, it’s pretty normal in the US to have family and friends at the defense. But it’s not absolutely ubiquitous, and I’ve known the expectations to vary even between different departments at a single university. So the best way to be sure is to ask around in the department — ideally, ask some faculty members who’ve been around long enough to see a few defenses — and confirm with them whether it’s usual to have family members there.

(In any case, it certainly shouldn’t affect the committee’s academic assessment of her; at worst, it would be seen as a bit of a social faux pas. But nobody wants to be worrying about social issues during their defense, and it can’t hurt to ask.)

In many parts of Europe and in Turkey (where I'm from), it is very common and normal - expected, even - for one's parents to attend their thesis defense.

The thesis defense is an important life event akin to one's graduation, marriage, etc. Especially when the event is designated as "open the to public," I think it would be weird if the parents did not attend.

My own parents attended my master's thesis defense (in Turkey), and they will surely attend my PhD defense as well. Practically all of my friends with PhDs have had their parents attend their defenses (in Turkey, Europe, and the US). The ones that defended without their parents being present did so because they were studying overseas, and their families could not make the trip.

In many cases the families not only attended, but brough cookies, pastries, etc. as well; as treats for the other guests and the committee. This in particular could be a bit unusual in the US, but it is common in Turkey and (I'm told) in Europe as well.

So, there are cultural differences in how this is handled, but there is definitely no universal norm that says to keep the family out of the thesis defense completely. In some cultures, the norm is for parents to attend.