It happens that "sister" (and "mother") are used for relationships between various inanimate entities - ships, companies, schools, monasteries, languages - and not "brother" or "father". This is simply a fact about English, with no obvious explanation.

I'm dubious that this has anything at all to do with the use of "she" for ships and countries, but I may be wrong.

It happens that "sister" (and "mother" and "daughter") are used for relationships between various inanimate entities - ships, companies, schools, monasteries, languages - and not "brother" or "father" or "son". This is simply a fact about English, with no obvious explanation.

I'm dubious that this has anything at all to do with the use of "she" for ships and countries, but I may be wrong.