Why use the term "Sister sites" instead of "Brother sites"? [duplicate]

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.

I suspect the terminology sister sites is derived from sister cities. Traditionally, cities, countries, and ships have been personified as feminine in English, although this is growing less common due to the influence of women's rights.