The 20th birthday is an important birthday in life. It is not only a coming-of-age ceremony, but also a time for you to be independent. I think you will have a grand birthday soon. Here are some suggestions:
First, give your sister what she wants, such as an MP3 or a trendy little gift. I believe that a 20-year-old girl will like it very much.
Second, give your sister a gift that she can use, such as her commonly used cosmetics, or a new bottle of perfume. As a coming-of-age gift, perfume must be a good gift and she will like it.
Third, a creative gift, give her a purple crystal music box, full of dreamy purple and nostalgic music. As a commemoration of childhood, your sister will like it very much. You can also give her a big teddy bear. The cute shy bear is a gift that all girls will like very much.