Teenage girls prefer beautiful and romantic things, as well as gifts such as teddy bears.
I recommend a few gifts to you, such as a big stuffed bear. She must like cute gifts at this age, and this teddy bear just meets this characteristic. Also, girls of this age generally start to love dressing themselves up. You can give them some beautiful hairpins, necklaces, bracelets, etc., and they will definitely treasure them.
Recently popular gifts like the sun jar of the twelve zodiac signs and the crystal music box are all good gifts. You can also give her a brand new school bag, pen holder, etc. New school supplies will also motivate her to study hard. I think you won't miss this gift.
No matter what gift you give to your sister, it represents your thoughts about her. If you want to give her a great gift, I believe you still know your sister very well. Believe in yourself, she will definitely like the gift you buy. I wish your sister a happy birthday and become more and more beautiful.