why did the peasant family raise elizabeth and why did they give her up?

Elizabeth's parents were alive, and she was living with a penniless Italian family. As Elizabeth was an orphan, and her adoptive family lacked the means to care for her, the Frankensteins determined to raise the child as their own. The child, whose name was Elizabeth Lavenza, became Victor's sister and his constant companion, as well as the object of his unquestioning worship. For him, she is his most beautiful, most valuable possession.