Considering that the nations are described as “odd beings”,
many aspects of their existence are either left open-ended or unanswered. It’s probably
meant to be like this, as it allows Hima to have more creative leeway.

On the other hand, there’s a consistent logic in the process
of how the nations are born that I’d like to go over.

How Are They Born?

For the most part, the nations are born as babies.

Ex: Egypt and his mother, Ancient Egypt.

This excludes…

1) The micronations, who are noted to be anomalies outside
the rules of nationhood.

2) Germany, who’s born as a young child given that he
already had an existing body (I’ll get back to this in a moment).  

We know from Iceland that the nations spontaneously gain
consciousness and come into existence. It’s not a formal birth process. They’re also instinctively able to tell that they’re different from humans. 

What signifies them? 

What signifies a nation as a baby is the nightgown they
wear. Personally, I don’t believe that the nightgown has any real significance
other than the fact that it acts as a plot device to distinguish the nation’s
young age.

How do they know who they’re related to?

Just like the nations are able to sense each other, the baby
nation is also able to sense their biological sibling.

This is seen with Iceland when he’s born. He sensed that
Norway was his brother upon gaining consciousness.

Norway also senses Iceland.

Likewise, we also know that later on, Iceland takes a DNA
test–in this case, it’s a land excavation–to confirm that there were no
Icelandic natives. Instead, the first people to settle in Iceland were
Norwegian.

That’s why Iceland and Norway are related.

On the other hand, America was colonized by several nations at once, not to mention that there
were already native tribes living there. As such, he’s referred to as an “ingredient.” He represents a mix of several cultures. [x] 

Further, America doesn’t ‘sense’ an older brother from the pool of countries colonizing him. Instead,
Finland, France, and England deliberate over who will assume the role of America’s older brother.

It’s also important to mention that “onii-chan” in Japanese
does not always literally mean older brother. It can be an expression of “endearment” or “adoration.”

There are different cultural connotations that are
unfortunately lost in direct translations.

Despite this, America does, in fact, have a biological brother: Canada [x]

Germany’s ‘Existing Body’:

As mentioned above, most nations are born as babies. Germany
is an exception to this rule as he’s born as a young child…or is he?

Disclaimer, I’m going
outside canon now. This next part is theory based on canon fact.

Many people have speculated that Germany’s ‘existing’ body
means that while Holy Rome died, his body remained. Germany then resumed this
body after German Confederation.

The ‘existing’ body, then, would be the fact that Germany’s spirit
inhabited Holy Rome’s old body, hence the reason why he wasn’t born as a baby.

This perspective doesn’t make sense to me, as we know that
it’s possible for nations to live past their date of dissolution. More
importantly, Holy Rome lived past his dissolution.

Another problem with this perspective is that the nations’
bodies don’t remain when they die, but rather, they fade away. The nations are
personality-based, meaning that when their culture dies, so do they. [x]

Put another way, culture doesn’t die definitively. It fades
with time – A nation’s path to death is reflective of this.

This would explain why Holy Rome didn’t die right away, albeit being ill. The
German culture he represented was still alive and well. 

Meanwhile, my take on Germany’s ‘existing body’ is vastly
different. I’m of the mindset that Holy Rome lost all his memories when he
became Germany. [post on that here].

In other words,
Germany wasn’t born as a baby because he was already existing, as in he had a living body.

In sum, while many questions about the nation’s existence
are left unanswered, there is a relative level of consistency regarding the
ages in which nations are born as well as the role that evolutionary instinct
plays in how they find their relatives.