祝妹妹高考顺利的英文表达可以有以下几种：

Good luck on your Gaokao.

Good luck for your exam and do your best.

You can do this. Best of luck.

It’s your lucky day.

Don’t worry. I’ll keep my fingers crossed for you.

To surpass oneself, to challenge oneself, to challenge weakness, to challenge laziness, to challenge bad habits. （这句话鼓励妹妹超越自己，挑战自我，克服弱点，改掉坏习惯）

Every day is a new beginning, every day brings a little progress, every day brings a little harvest! （这句话提醒妹妹每天都是新的开始，每天都有进步和收获）

The college entrance examination is coming soon, learning is relaxed and time is organized. I wish you a good result. （这句话提醒妹妹高考即将来临，学习要有张有弛，时间安排有序，并祝愿她取得好成绩）

Never give up, and good luck will find you. （这句话鼓励妹妹不要放弃，好运总会到来）

If not us, who？ If not now, when？ （这句话激励妹妹要把握现在，不要犹豫）

Life doesn’t get easier, you just get stronger. （这句话告诉妹妹生活不会变轻松，但她会变得更强大）

I wish you success in the college entrance exam.

I wish you the best of luck in the college entrance exam.

Book mountain has a path, hard work is the key, ten years of cold windows, only look at today. Ordinary mentality, extraordinary performance. The brush strokes are like magic, sleep is sufficient, energy is abundant. Gold list name, who else but you! （这句话用诗一般的语言祝愿妹妹高考顺利，发挥出色）

希望这些祝福语能够帮助妹妹在高考中取得理想的成绩!