oh and I remember, during college days at the Islamic meetings - the top lady and the top guy - who organized all Islamic activities etc. called each other brother/sister. however, after some time when I went to one of the meeting - they were being congratulated on their engagement!
:D I know of a 'head brother' and 'head sister' marriage too!
However, I guess its just out of respect that some muslims who are unrelated to each refer to each other as brother and sister.
Wow, im being so politically correct today. Will have to make up for it tomorrow by calling someone a paki.