It's your sister's birthday, of course she won't want you to spend money to buy her a gift, but who doesn't want to receive a gift on their birthday? So you can prepare it secretly, which can be regarded as a small surprise for her. Here are some suggestions for you:
First, since she is your own sister, you should know very well what she wants in her daily life. If it is within your control, you can buy it for her, and she will be very surprised.
Second, give her a creative gift, such as a beautiful crystal music box, or a color-changing water cup that you and her can share, or even make a pair of real dolls of you, which will make her very touched. It mainly depends on your intention.
Third, give some gifts that girls like, such as crystal pendants, crystal ornaments, or small gifts such as perfume, and she will love them.