Into the Water by Paula Hawkins

Just days before her sister plunged to her death, Jules ignored her call.
Now Nel is dead. They say she jumped. And Jules must return to her sister's house to care for her daughter, and to face the mystery of Nel's death.
But Jules is afraid. Of her long-buried memories, of the old Mill House, of this small town that is drowning in secrecy . . .
And of knowing that Nel would never have jumped.