I think it's striking that Nelly implies Mr. Earnshaw is jealous of Heathcliff's fondness for Catherine. Because jealousy from Mr. Earnshaw in this context is... strange. It makes sense that he would be angry if Heathcliff refused to obey him and prioritised pleasing Cathy instead. But the language Nelly uses specifically suggests jealousy - "showing how her pretended insolence had more power over Heathcliff than his kindness: how the boy would do her bidding in anything and his only when it suited his own inclination."
An adult feeling angry because their child is misbehaving and refusing to obey them in order to please a peer is normal. An adult feeling jealous that their child is more motivated to please a peer than to please them is not. An adult might be jealous if their child preferred another adult; a child might be jealous if their friend preferred another child. But an adult being jealous because their child prefers a same-age peer is kind of... weird? Emotionally immature? Of course you are not your twelve-year-old's friend, that's not what a parent-child relationship is. And it's particularly weird when it involves being jealous of your biological child because your foster child likes her more. Kind of drives home the extent to which Heathcliff is the favourite. Earnshaw is not jealous of him for having Cathy's love and regard!
Of course you can argue that most children don't actually love their friends more than their parents, even if they like being around them more. Whereas Heathcliff actually does love Catherine more than her father, so the situation is different. But it also reminds me a little of the way Graeme Tytler describes Cathy Linton's relationship with Edgar as "almost incestuous." Which is maybe an unfortunate term to use, because it implies a sexual undercurrent to the relationship that is clearly not there either with Edgar and his daughter, or with Mr. Earnshaw and Heathcliff. But I think with both relationships there's a sense of... an attachment that is very intense, perhaps too intense, in a way that is damaging.
I wonder if Mr. Earnshaw's possessive attitude towards Heathcliff also contributed to Hindley's cruelty towards him. In the sense that - "be nice to the orphan child your father has taken in," is a reasonable expectation. But "if the orphan child your father has taken in likes you better than your father, your father will get jealous of you," - that is a very weird emotional situation for a fourteen-year-old to handle.
Also. If you adopted an orphan child, and almost everyone else in the household hated and mistreated him, and he had a good relationship with one other person in the house, it would seem reasonable to like that good relationship and encourage it. There's something quite selfish about Earnshaw being jealous about Heathcliff and Cathy's relationship when Heathcliff has so few good relationships within the household.
And given Mr. Earnshaw's jealously here, I wonder if on a subconscious level his obvious favouritism towards Heathcliff is motivated partly by the desire to drive a wedge between Heathcliff and the other household members. If they remain hostile to him then Heathcliff will always be his alone rather than belonging to the family, and will also be a symbol of his authority within the household and a symbol of his moral righteousness in comparison with his cruel family members who refused to accept the poor fatherless child.
And of course Heathcliff is being sarcastic when he says this, but:
