English works with both metaphors but apparently prefers use of the feminine personifications. It’s your mother country, your sister city, etc. I suspect that the exceptions are motivated in large part by something quite similar to MADAME’s suggestion.
However, it’s interesting to note that a large portion of the uses one runs into of someone referencing a “kinship-term political unit,” it is a translation from another national language, as, here, Norwegian. French, German, Russian: all have standard metaphorical uses that tie into the national cultural norms. Die Vaterland means something different than “mother country,” and the difference is not just in the gender of the metaphorical relative.
And with the strong anti-war sentiment of the Scandinavian nations generally, I think that probably “brother nations” is merely a Norse usage literally translated.