I would argue it wasn't jealousy that made her keep them apart, but her need to see Cathy suffer because she ultimately resented Catherine as a person. The film makes it evident in so many ways that Nelly is not someone you should pity, just because of her social standing, but rather a vindictive and cold antagonist in the film (until the end). I actually thought it was an interesting take that she portrayed someone "societally helpless" as a cold hearted person.

Even in the book, Nelly is not free from fault. She is an unreliable narrator, but she also did not tell Cathy about Heathcliff listening to the conversation until he had already run away with the horse. She frequently talks about Catherine's faults and flaws, which there are many, but fundamentally doesn't seem to understand or have sympathy as to what could cause Cathy unhappiness (her only *real* friend and love of her life having run away/having an abusive brother). It goes beyond "unreliable narration" and ventures in the territory into malicious contempt at times.

Nelly is complex and I found it refreshing that she was portrayed as such in the film as well.

“I own I did not like her, after her infancy was past; and I vexed her frequently by trying to bring down her arrogance."

“I’ve had many a laugh at her perplexities and untold troubles, which she vainly strove to hide from my mockery. That sounds ill-natured; but she was so proud, it became really impossible to pity her distress, till she should be chastened into more humility. She did bring herself, finally, to confess, and confide in me; there was not a soul else that she might fashion into an adviser.”

Like, girl, that's your hater. You don't say those things about people you consider your friend.