Why is Mattie "indentured" to Ethan and Zeena?

From the text:

Mattie Silver was the daughter of a cousin of Zenobia Frome's, who had inflamed his clan with mingled sentiments of envy and admiration by descending from the hills to Connecticut, where he had married a Stamford girl and succeeded to her father's thriving "drug" business. Unhappily Orin Silver, a man of far-reaching aims, had died too soon to prove that the end justifies the means. His accounts revealed merely what the means had been; and these were such that it was fortunate for his wife and daughter that his books were examined only after his impressive funeral. His wife died of the disclosure, and Mattie, at twenty, was left alone to make her way on the fifty dollars obtained from the sale of her piano. For this purpose her equipment, though varied, was inadequate. She could trim a hat, make molasses candy, recite "Curfew shall not ring to-night," and play "The Lost Chord" and a pot-pourri from "Carmen." When she tried to extend the field of her activities in the direction of stenography and book-keeping her health broke down, and six months on her feet behind the counter of a department store did not tend to restore it. Her nearest relations had been induced to place their savings in her father's hands, and though, after his death, they ungrudgingly acquitted themselves of the Christian duty of returning good for evil by giving his daughter all the advice at their disposal, they could hardly be expected to supplement it by material aid. But when Zenobia's doctor recommended her looking about for some one to help her with the house-work the clan instantly saw the chance of exacting a compensation from Mattie. Zenobia, though doubtful of the girl's efficiency, was tempted by the freedom to find fault without much risk of losing her; and so Mattie came to Starkfield.