Elder sister姐姐 elder sister你就叫她姐姐sister,女孩子不喜欢长大。"you are my big sister, I am your little sister ."----it shows your humbleness and willingness."sister ! may I call you sister ?"----姊妹, 亲切.or , just call her by her name. generally it is not considered rude to call a person by name .for example, the classmates of my son come visiting in my house, they can either call me Mr.张, or call me 大口 , comfortably. (if my name is 张大口.)英语国家就是用big sister 口语化妹妹用baby sister or litter sisterelder这个词正式化sisteror big sisteror sis