according to one of the comments to the above mentioned post,
Friendship with a girl (however platonic it may be), is perceived to come about because the girl finds you fun to hang out with, smart, and a cool company in general.. The moment 'brother' comes into picture, it seems to throw all those flattering adjectives out of the window.. no idea why it happens though.. Also, 'brother' forcefully closes the door to a lot of talk-worthy topics and that occasional harmless, playful flirting between friends of opposite sex, which I think is healthy for a normal male-female friendship..the indian context also has the 'rakhi brother', which apparently is commonly used to keep unwelcome advances at a distance...
i agree with the above comment, and i think this has a lot to do with the way brother-sister relationships are viewed (particularly in india perhaps). i grew up seeing my younger brother more as a playmate than a sibling. close enough in age, we did many things together, shared many interests and a sense of humour. this continued until today. many a time, my girl friends would remark on the fact that we could finish each other's sentences or that he was privy to my crushes. he was my best friend, who also happened to be my brother--seemed totally normal to both of us! my relationship with my older brother is far more conventional, due to various factors i guess, age difference perhaps being the biggest.
the point i want to make i guess, is that it really depends on the kind of relationship one has--while there is no playful flirting going on between me and my younger brother, he is much more my best friend than simply my brother. the few good guy friends i do have now however--again without any possibility of romance/sex on the table--are also not in the 'brother' category though..