need help finding book

[Juniper]

I need help finding a book I read years ago that i cant remember the name or author of. Heres a summary *SPOILERS* the book follows this girl who's older sister went missing years ago but then comes back home and is obviously different. The book is told from the girls perspective and what we believe to be the older sisters when she is kidnapped but it turns out the girl actually killed her sister and the girl who claims to be her is just someone who looks almost identical to her that was kidnapped from a young age and saw the opportunity to get out by posing as this girls sister. in the end the two make a pact to keep each others secret and live as sisters.