网上老外的回答：
Both are correct. If the asker is referring to your actual sister（or brother）, then the usual response would be "Yes, she is." If the asker is referring to a photograph of your sister, then it is not uncommon to for someone to respond "Yes, it is." Perhaps, subconsciously, one may be mapping "it" to the photograph which is an "it" instead of to the person portrayed, which is a "she". Note there are similar situations in which 'it' might be used to refer to a person. For example: "Who's that knocking on the door?" "It is Bob, our neighbor from across the street."
在不确定的情况下，用he is会安全点
