あなたの妹はあなたが10歳のときに半年でした。つまり、彼女は当時5歳でした。それ以来、30年が経過しました。つまり、彼女は現在35歳です。
Your sister was half your age when you were 10, which means that she was 5 years old at the time. 30 Years have passed since then, which means that she is now 35.