The Alington Inheritance by Patricia Wentworth
Jenny is 18 when her guardian is killed and her wealthy relations the Forbes invite her to stay. Consoled by the affectionate attentions of their little girls and the charming elder son, she starts to find her feet again.
Then she overhears a private conversation and realises she is in terrible danger. Terrified and bewildered, Jenny flees from the village, desperately hoping her enemies will forget all about her. But she's wrong . . .