Is your family having a Sweet 16 for your sister? If so, she will most likely receive money as gifts from the guests, so i'd suggest a real "gift" from you. One that down the road, years from now she will still have (money gets spent and then it's gone), she can look at, and remember that her brother gave it to her. Something like a nice piece of jewelry, earrings, a necklace or a bracelet.
My niece will turn 16 in a couple of years, and if she keeps going the way she is, I will probably have to get her a storage locker to hold all of her academic awards.
You could get her her very own late-night talk show. She'll be the envy of all her friends (until the network cancels her, anyway).