The Twin's Daughter by Lauren Baratz-Logsted is an historical fiction mystery for young adult readers. It really has all the necessary ingredients for an enjoyable read -- especially the finely spun web of mystery.
The story is told in first person narrative from the viewpoint of Lucy. She is born into a wealthy family. Her father is a writer who has no need to work -- his family has more money than they will ever be able to spend -- so he spends his days in his study writing books that may never be published. Her mother dotes on Lucy, who is her only child, having suffered through two miscarriages.
One afternoon, Lucy answers the door, and the woman on the steps is an exact replica of her mother. It turns out that her mother was adopted (a surprise) when a maid became pregnant, but when twins were born, the wealthy couple only wanted one child so the other went to an orphanage.
Helen, the sister, is taken into the household, educated and dressed. She and Lucy become close. However, mysterious happenings are taking place. Lucy sees someone -- either her mother or Helen -- sitting on a park bench with an obviously lower-class man talking heatedly. On her return, Helen denies having seen anyone.
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