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.

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.

.sis is already taken, it is (was?) the package files for Symbian. Basically .apk before Android, if you'd like.