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:
It's embarrassed also in Japan for "pregnant vs. fat", but "imouto vs. ane" is not the case, I believe. Of course, it depends on the situation(generation, male/female, the difference in age, etc.), though.