Thank you 😊! Here’s a basic rundown of the most relevant strips pertaining to Portugal. I’m not sure if there would be enough on him to do a full characterization post given how recent he is in the canon verse.

Portugal’s introduction can be found in chapters 77-80, and 83-84 of World Stars.
It covers his relationship with Spain, the Netherlands, Macau, Japan, and England. [x]

Ex: He had a close relationship with England, Macau was his underling, he had rocky trading relations with the Netherlands over Macau, and he had good trading relations with Japan. There’s also his hilarious teasing/ somewhat belittling relationship with Spain. 

Here’s part of his first character note:

Now for your question:

I know for a fact that he’s not related to Spain. They’re simply listed as neighbors with a similar ancestral influence.

They’re noted to be similar in appearance, but not once are they ever mentioned to be brothers. They even recognize this upon being mistaken for each other. 

Given the context, if they were brothers, they would have attributed the similarity of their looks to a blood-relation. They don’t, so it would follow that they’re not related.