in nowadays,between two girls,they may call each other belle(美女mei nv)、sibling(姐们儿jie men er). the sibling means like buddy. or when they were against each other and they were closed enough, they can call big-sister(大姐da jie) for badinage . between two boy: usually,they call each other buddy(哥们儿ge men er),this word is used widely. now,among the young pepole,neither you are bigger or smaller,you can both call him brother(哥哥ge ge), of course it is badinage. between a girl and a boy,neither who was bigger on age,if they were closed enough,they can call each other brother(哥哥ge ge) or sisiter(姐姐jie jie) or big-sister(大姐da jie).if they was not close enough,the best way is call friend(朋友peng you)、Mr、Miss or name. if you don't know his(her) name,you can call buddy(哥们儿ge men er)、belle(美女mei nv). wants they could help