Little Sister Song by Sara Creasy is book 1 of the Wynter Wild series. It was available on Wattpad from December 2018. It was published March 2019.
The book introduces the characters of Wynter Wild and her four older half-siblings Caleb, Indio, Joy, and Jesse Fairn. Wynter (14) escapes the ashram in Arizona where she grew up with her sister Joy (22) in a cult-like religion called the Light. Caleb takes Wynter into his home after she shows up in Seattle alone a few days after New Year 2013. He and his brothers never knew they had a younger half-sister.
Jesse teaches Wynter how to live on the outside, although his lessons are biased toward his own interests of philosophy and science. Indio, who lives in Portland, bonds with her over their shared musical talent. Caleb tries to persuade Joy to take custody of Wynter, but she seems unable to function outside the the ashram.
Books in the Wynter Wild series:
02 Out of Tune
08 Minor Key