She's a clever girl, but she's completely overshadowed by her extremely bright big brother. Not only does he do everything first because he's 2y older than her, but he does everything 'better', too. She is suffering from classic middle-child syndrome, because she also has a very demanding little brother.
How can I help her? How can I help her bring out her own talents, rather than just follow in her brother's footsteps? For example, ds1 learns violin, so dd is really looking forward to learning violin - but perhaps she would be better off learning a different instrument for the sake of her own individuality? Also so that if she turns out not to be particularly good at it, it won't be just one more thing that ds1 is better at doing than she is.
Please don't turn this into an argument about the G&T label. It's a reality, however you describe it, that some children are unusually bright, and this brings problems of its own.