Ещё раз is 'one more time' and thus it doesn't mean the same as 'again'. Опять is less literary than снова and is often used in the negative meaning, for example: 'Ты опять не сделал домашнее задание!'
My complaint about this sentence is that it seems to portray a narration of an event. It seems like it's trying to say, "My sister is opening the book again." Or is that yet another conjugation I've yet to learn?