How does Zillah’s relationship with Cathy and Hareton in Chapter 30 parallel Nelly’s relationship with the elder Catherine and Heathcliff?

Outside of the fact that Zillah acts as a go between for the two, there are few similarities. Nelly loved Catherine and Heathcliff.... she had a vested interest in the family. Zillah has no such vested interest.... she doesn't even like Cathy.