あなたが4歳で、あなたの妹があなたの年齢の半分だったら、彼女は4/2 = 2歳だったでしょう。今、あなたが28歳の場合、彼女はあなたより28-26 = 2歳若いので、彼女は28-2 = 26歳になります。
If you were 4 years old and your sister was half your age then she would have been 4/2 = 2 years old. Now if you are 28 years old, she would be 28-26 = 2 years younger than you, so she would be 28-2 = 26 years old.