Is she into Abe books and cds? cos then you can borrow them when Shes's done. haha. Or any other life affriming books and ideas to help her along in life. Not a more loving gift I reckon xx
Since I do not know your sister and have not had to pay much attention to the needs and desires of 21 year old girls in a very long time - though I remember the days I did fondly ;-) I do not think I can help you in actual ideas. But I love giving gifts to people and I get so much pleasure from finding a wonderful meaningful gift for people and many have commented that I'm very good at picking them and it is not often that I find it difficult to pick gifts for people.
So my advice is get excited about the opportunity. And then really think about and appreciate her and think about thing like, things she has told you, things that make her laugh and smile, things she gets excited about, even things she has mentioned in passing and I'm sure a perfect gift idea will present itself.
I have also asked the universe for help to inspire me or to put the perfect gift on my path when I'm struggling and the inspiration or solution always comes.
I believe the secret is always to get excited at the opportunity to express your love and appreciation in the form of a gift and then the gift giving gets easy and fun and so it flows easily. That after all is the secret of life, get excited about prospects, have an intention of spreading and sharing joy and good things are sure to flow.