After watching the 1970 version of Wuthering Heights, I was struck by Mrs. Earnshaw's comments which suggested that Heathcliff might be Mr. Earnshaw's illegal child. This is a daring interpretation, in my opinion, which I don't notice in other adaptations - but the 1970 version also hints that Nelly had a crush on Hindley and that Cathy's child might be Heathcliff's.
In the book (chapter 4), Emily Bronte wrote Mrs. Earnshaw's reaction after his husband came home from Liverpool bringing Heathcliff was as follows: " Mrs. Earnshaw was ready to fling it out of doors: she did fly up, asking how he could fashion to bring that gypsy brat into the house, when they had their own bairns to feed and fend for? What he meant to do with it, and whether he were mad?"
Perhaps the 1970 version used this for their reason: "He took to Heathcliff strangely, believing all he said (for that matter, he said precious little, and generally the truth), and petting him up far above Cathy, who was too mischievous and wayward for a favourite."
How could a father prefer a stranger he met on streets than his own daughter?
Transcript from 1970 version
Mr. Earnshaw: Now, this isn't exactly what you've been expecting, but let's say it's a gift from God.
Mrs. Earnshaw: Looks more like a gift from the devil. I'll not have gypsies in my house.
Mr. Earnshaw: He's not a gypsy.
Mrs. Earnshaw: I don't care what it is. Get shot of it.
Mr. Earnshaw: What? And let him starve?
Mrs. Earnshaw: Why not? Plenty more do. What's so special about him that makes you so fine and tender?
Mr. Earnshaw: Nothing, except I found him in Liverpool without a soul to-
Mrs. Earnshaw: No doubt found more besides. There's no need to drag your doings back here.
Mr. Earnshaw: You're too clever for me by half.
Mrs. Earnshaw: You're not clever enough. What'll you do? Make him work?
Mr. Earnshaw: Aye, but no more than others. We lost a son, didn't we? Thanks be to God, we have another. He can be a brother to them.
Mrs. Earnshaw: I've no doubt he is already.
[Also interesting to note that this version shows how Hindley's new fiddle was ruined by Heathcliff. For a child, this was a strong reason to hate Heathcliff for the rest of his life.]
Compare that with some other versions below:
In this version, the introduction was by the gate of Wuthering Heights, where Mr. Earnshaw met the family doctor.
Mr. Earnshaw: A gift from God; though he is dark as if he came from the devil. [...] I find him starving in the streets of Liverpool, kicked and bruised and almost dead.
Dr. Kenneth: So you kidnap him.
Mr. Earnshaw: Not until I try to find out who his owner was, but nobody would lay claim to him. So long as he shown allegiance that he was, I brought him home.
[Hindley got his fiddle. But he had to share his room with Heathcliff. That's a sensible reason to hate Heathcliff, if Hindley had never shared anything before in his life.]
Mr. Earnshaw: I found him starving in the streets of Liverpool.
Hindley: He's a filthy gypsy, Father.
Mr. Earnshaw: He's a gift from God. You're to treat him as your new brother.
Cathy : But where's my present?
Nelly: Hasn't he got any family of his own?
Mr. Earnshaw: He's part of our family now. (To Heathcliff) That's your brother Hindley and this is your new sister Cathy.
[I think in this version Hindley hates Heathcliff only because Heathcliff is dirty and a minority.]
Heathcliff - The Musical
Mr. Earnshaw: Here is your Christmas gift. A gift from God, Cathy, although he 's dark as the devil. Call him Heathcliff. Hindley, he's your brother now.
And from the Gypsy Bundle song:
No history, no owner, no rhyme, no reason.
Nor explanation of his sorry state.
*From all these 4 versions, Mrs. Earnshaw only appeared in the 1970 version.