姉妹ですか？ (Shimai desu ka?) if you're asking a girl.How would you ask the following question in Japanese:
"Is she your sister?".
As far as I can see, there is only a word for 'older sister' or 'younger sister'.
From U.S. perspective if you ask somebody if they are older but they are really younger then it's not a good feeling sometimes. Like if you ask somebody when they will have a baby but you find out they are not having a baby, they are just fat. :facepalm:About age? or not sister? I don't think it's rude, anyway.