Actually, insulting your sibling may not be a sign of inferiority. It is very, and I do mean very common, for siblings to insult or even constantly insult each other. My little brother falls me stupid sissy all the time, I call my little sister shorty. And none of us have the issue. It would be very weird if we didn’t us them at this point. And, North and South Italy have special insults for each other, can’t remember the exact words but the insult could be to show the divide and normal sibling

[On the topic of
Romano referring to Veneziano as stupid/ idiot little brother]

While that may be the
case for most siblings, Romano doesn’t use insults to show his affection. I
know it’s nice to imagine Romano and Veneziano to be like that, but from what
we’ve seen in the manga, they just don’t have that type of relationship dynamic.

You work from what’s
seen in the strips, not what you interpret a character to be like – especially if
canon has already proven the contrary.

If anything, Romano uses actions, regardless
of the insults that fly out of his mouth, to show his affection.

Ex: He’s still calling
Spain an asshole or bastard as he sends imports to help with the Spanish
economic recession.

On the other hand,
with Veneziano, Romano doesn’t ever use insults to refer to the latter in an
affectionate way. They don’t have a good face-to-face relationship, and there’s
a ton of hostility and resentment on Romano’s side of things. In this case,
Romano’s insults do in fact have to do with inferiority.

The suddenness of
their unification is noted to have caused a tension between them.

Romano is also noted
to detest North Italy “a bit.”

As such, he has a much
better relationship with Spain than he does with Veneziano.

Romano’s inferiority
complex is most seen when he uses insults to undermine Veneziano’s accomplishments.

Ex: Veneziano’s
engineering feats during the Industrial Revolution.

It’s like this in
present strips too. The complex hasn’t gone away.

In the Annoy Romano
series, one of the prompts is to be surrounded by Veneziano, his mochi, and his
nekotalia counterpart.

Similarly, Romano
hates to be referred to as Italy’s older brother, as it lacks individual
differentiation.

Again, his insults toward
Veneziano don’t possess any affection to them. Romano refuses to work with Veneziano.

To put things more into perspective, Romano’s hostility of Veneziano is bad enough for Rome to visit him in his dreams, chiding him for this.

Of course, this doesn’t
mean that Romano doesn’t care about Veneziano either. He’s just not direct with
it. 

Ex: He looks after Veneziano during WW2, regardless if he degrades him while doing it.

Like I said, they lack a solid face-to-face relationship.