"Sister city" "Brother city" "Sibling city" "Twin city"

For some strange reason, no one ever says "brother city." The term "sister city" is very common. "Twin city" is also conventionally used. I do think it's a bit strange to say "sister city" but not "brother city," and it forces me to think about gender in a context where I do not want to think about gender. So I like the term "twin city" better. "Sibling repo" preferred to "sister repo" for the same reason (if I was reading this Github documentation, having to think about a repo as female gendered would be a highly distracting diversion while I'm trying to concentrate on source code).