The part of this passage from chapter 6 of Emily Brontë's "Wuthering Heights" that illustrates that Hindley’s wife did not belong to the upper class is:
"What she was, and where she was born, he never informed us: probably, she had neither money nor name to recommend her, or he would scarcely have kept the union from his father"
Hindley's choice to not reveal his wife's background shows that she does not come from a wealthy or important family, because if she was part of the upper class, he would not have had reasons to hide their marriage from his father and all the other people in his life.