The Stillwater Girls by Minka Kent

Two sisters raised in fear are about to find out why in a chilling novel of psychological suspense from the author of The Thinnest Air.
Ignorant of civilization and cautioned against its evils, nineteen-year-old Wren and her two sisters, Sage and Evie, were raised in off-the-grid isolation in a primitive cabin in upstate New York. When the youngest grows gravely ill, their mother leaves with the child to get help from a nearby town. And they never return.
As months pass, hope vanishes. Supplies are low. Livestock are dying. A brutal winter is bearing down. Then comes the stranger. He claims to be looking for the girls' mother, and he's not leaving without them.
To escape, Wren and her sister must break the rule they've grown up with: never go beyond the forest.
Past the thicket of dread, they come upon a house on the other side of the pines. This is where Wren and Sage must confront something more chilling than the unknowable. They'll discover what's been hidden from them, what they're running from, and the secrets that have left them in the dark their entire lives.