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doppelganger some 5-10 yrs back it was not AT ALL WEIRD...Weirdness came in when the West proclaimed it weird...because by calling someone sister automatically maeant respecting her...
Here in the West, a slightly religious person (be he Christian or Muslim) will call a girl sister just so she knows he doesnt have any other intentions except to talk to her in a polite manner... i.e. he is not hitting on her but genuinely want to ask her something...And in return the girl would answer Brother...it does not mean
blood relations but it just gives ease of mind....
Yes, society dictates stuff to people....
Some people are ready to say their ancestors were monkeys but it kills to call another bro or sis?!
Biologically we are come from the same ancestory and many religions call them Adam and Eve or Adam and Hawa or somewhat name close to that...if a stretched version of that rule makes someone say sis or bro...I dont see any problems...