In the Victorian Era, social class was not solely dependent upon the amount of money a person had; rather, the source of income, birth, and family connections played a major role in determining one's position in society. And, significantly, most people accepted their place in the hierarchy. In addition to money, manners, speech, clothing, education, and values revealed a person's class. The three main classes were the elite class, the middle class, and the working class. Further divisions existed within these three class distinctions.
The characters in Wuthering Heights demonstrate the nature of this class-structured society. The Lintons were the most elite family in the novel, and Thrushcross Grange was a superior property to Wuthering Heights, yet they were not members of the uppercrust of society; rather, they were the professional middle class.
Although Wuthering Heights was a farmhouse, the Earnshaws were not members of the working class because they were landowners who had servants. Their station in society was below the Lintons but not significantly below. Nelly, a servant of the Earnshaws, represents the lower middle class — those who worked non-manual labor. Servants were superior to manual laborers, which explains the problems created by Heathcliff.
Heathcliff is an orphan; therefore, his station is below everyone else in Wuthering Heights. It was unheard of to raise someone from the working class as a member of the middle-to-upper middle class. Even Nelly, who was raised with the Earnshaw children, understood her place below her childhood friends. When Mr. Earnshaw elevates the status of Heathcliff, eventually favoring him to his own son, this goes against societal norms.
This combination of elevation and usurpation is why Hindley returns Heathcliff to his previous low station after the death of Mr. Earnshaw, and that is why Heathcliff relishes in the fact that Hindley's son Hareton is reduced to the level of a common, uneducated laborer. And social class must be the reason Catherine marries Edgar; she is attracted to the social comforts he can supply her. No other plausible explanation exists. Catherine naively thinks she can marry Edgar and then use her position and his money to assist Heathcliff, but that would never happen.
When Heathcliff returns, having money is not enough for Edgar to consider him a part of acceptable society. Heathcliff uses his role as the outcast to encourage Isabella's infatuation. The feelings that both Catherine and Isabella have for Heathcliff, the common laborer, cause them to lose favor with their brothers. Hindley and Edgar cannot accept the choices their sisters make and therefore, withdraw their love. When a woman betrays her class, she is betraying her family and her class — both unacceptable actions.