The Kingdom of Back by Marie Lu
Two siblings. Two brilliant talents. But only one Mozart. Born with a gift for music, Nannerl Mozart has just one wish - to be remembered forever. But even as she delights audiences with her masterful playing, she has little hope she'll ever become the acclaimed composer she longs to be. She is a young woman in 18th century Europe, and that means composing is forbidden to her. She will perform only until she reaches a marriageable age - her tyrannical father has made that much clear. And as Nannerl's hope grows dimmer with each passing year, the talents of her beloved younger brother, Wolfgang, only seem to shine brighter. His brilliance begins to eclipse her own, until one day a mysterious stranger from a magical land appears with an irresistible offer. He has the power to make her wish come true - but his help may cost her everything.