Nelly describes how Heathcliff came to be the owner of Wuthering Heights after Hindley Earnshaw mortgaged it away to pay off the debts he accumulated by gambling with Heathcliff. This is an instance of situational irony, given everything readers would expect about the importance of inheritance in old, respected families like the Earnshaws.

The guest was now the master of Wuthering Heights [...] Earnshaw had mortgaged every yard of land he owned for cash to supply his mania for gaming: and he, Heathcliff, was the mortgagee. In that manner, Hareton, who should now be the first gentleman in the neighbourhood, was reduced to a state of complete dependence on his father’s inveterate enemy; and lives in his own house as a servant deprived of the advantage of wages, and quite unable to right himself, because of his friendlessness, and his ignorance that he has been wronged.

Recall that when readers are introduced to Wuthering Heights, Lockwood notices the name of family ancestor "Hareton Earnshaw" above the door. Later, Hindley's son Hareton is named after his 16th-century forebear—a plain indication that Wuthering Heights is meant to be passed down to future generations of Earnshaws. Given the significance of inheritance in this society, then, it's an especially grievous irony that the current Hareton Earnshaw has been swindled out of what's rightfully his by Heathcliff—his father's poor, adopted orphan brother with no natural claim on the Earnshaw heritage.

This circumstance is also meant to show how truly dissipated Hindley became: allowing his son's inheritance to be gambled away is a shameful dereliction of duty. And after Hindley dies, young Hareton has no idea what he's lost because Heathcliff raises him the way Hindley raised Heathcliff —that is, like a servant, isolated and uneducated. So, even though Heathcliff is the one who abuses and degrades Hareton, he does so in retaliation for the way Hindley treated him, making Hindley responsible for the Earnshaws' downfall twice over.