Dear Arianna,
It's great that you and Gianna are best friends, but it's also understandable why the two of you want to be seen as separate individuals and not as "the twins." When people use that term—and they probably don't realize how much that bothers you—let them know in a tactful way that you'd prefer that they call you Arianna and your sister Gianna. To help people tell you apart, perhaps you could get to be known by the distinctive style that you prefer, and Gianna can find the style that she likes best as well. You might still be mistaken for your sister from time to time, so try to be patient as you explain who you are and then give them a tip that will help them remember who's who.