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This came up recently on programmers.SE, when someone (a Russian, presumably non-native English speaker) asked why we used the term "sister sites", instead of "brother sites". Of course, I'm a native English speaker, and I have no idea either, hence this question.
The full question, for context, was "Several of our sister sites have taken to featuring Questions [...]".
It does seem odd, considering that English (usually) uses masculine terms as the default.