I have noticed this quite often that other (closely) related common nouns are called sister [common noun]. For example:
This question is off-topic here, but is on-topic on our sister site.
This issue can't be resolved here. You will have to visit our sister branch.
Monopoly Casino Sister Sites are listed in here.
ABC is the sister company of DEF.
…and many more.
Why does this kind of phrases/sentences make use of the feminine gender, but never the masculine gender, like brother sites? Did this somehow remotely got influenced by feminism movements? Because looking at this Ngram I can see that such phrases were not present in the old times of 1900 A.D and before.
Any insight on this topic will be warmly welcomed.
P.S: I mean no ill-intention when asking this question or mentioning about feminism movement. This just piqued my interest.
Best Answer
The Oxford English Dictionary shows that this usage goes back to at least the 1500s. It provides the following definition:
Here are some examples of early usage:
This doesn't answer your question as to why "sister" is used, although if the usage was influenced by French or Latin, "church", "nation" and "university" are all feminine nouns in those languages.