I am a man, but I don't think that's why I think it. I'd be perfectly fine with a ".girl" or ".chick" or ".fem" or ".sis" extension, or any similar variation, if it happened to be a valid acronym for some technology.
Unless the issue is that "bro" has some other kind of objectively offensive connotations, which I completely disagree with. Many women (and some men) use "sis" (or, much more rarely, "dudette"), in the exact same way as "bro".
You pretty much can't win in that culture. You're a man, so you can't even grasp what it's like to be a woman, but trust us, it's horrible. And if you wouldn't mind if it were "sis", that's because you haven't been oppressed by women.
I understand there are gender issues, but if you have to overcorrect so much that you're changing an abbreviation to avoid the short form of "brother", you've gone wrong.
I am very thankful that my culture doesn't have this gender hostility and easy offense that's prevalent mostly in the US.
There are some common connotations for the type of people who call each other "bro", e.g. the muscle-bound narcissist who still smells like last night's kegger, where he got to feel up three different blackout-drunk girls. Of course, that isn't actually representative of the population of bro-sayers, so it's just as much a bigoted stereotype as anything else is.
I've actually known some stereotypical sorority-ish girls who use "sis" / "sister" in a very parallel way (shallow, narcissistic, petty, immoral). Doesn't somehow invalidate all use of the word. (And obviously not all sororities are like that, hence "stereotypical". Nor are all fraternities.)
