The Twin's Daughter by Lauren Baratz-Logsted

Lucy Sexton is stunned when a disheveled woman appears at the door one day…a woman who bears an uncanny resemblance to Lucy's own beautiful mother. It turns out the two women are identical twins, separated at birth, and raised in dramatically different circumstances. Lucy's mother quickly resolves to give her less fortunate sister the kind of life she has never known. And the transformation in Aunt Helen is indeed remarkable. But when Helen begins to imitate her sister in every way, even Lucy isn't sure at times which twin is which. Can Helen really be trusted, or does her sweet face mask a chilling agenda?

Filled with shocking twists and turns, THE TWIN'S DAUGHTER is an engrossing gothic novel of betrayal, jealousy, and treacherous secrets that will keep you guessing to the very end.

https://www.goodreads.com/book/show/7823311-the-twin-s-daughter