あなたが14歳のとき、あなたは14/2 = 7歳だった妹の2倍の年齢でした。
だから今、あなたの妹は14歳なので、あなたは14 + 7 = 21歳でなければなりません。
When you were 14, you were twice as old as your sister, who was 14/2 = 7 years old.
So now, since your sister is 14 years old, you must be 14 + 7 = 21 years old.