From the female servant at Wuthering Heights, Ellen found out that Heathcliff didn’t like Linton and treated him well only because he was his son. It ceased all the sympathy Ellen had toward him.
That female servant left two years after and was replaced by Zillah, meaning this happened approximately just two years ago.
One time when she was going out with Ellen, Cathy ran into Heathcliff, who persuaded her to come to Wuthering Heights to see her cousin. Although she did not recognize Linton at first, Cathy grew to love him quickly. She wondered how cousins could live only four miles apart without their knowing each other, so Heathcliff told her that he and Edgar had a serious argument.
Heathcliff was relieved that Cathy showed more interest in Linton than she did in Hareton. He admitted he would’ve liked Hareton even more if he weren’t in the way of his plan to marry his son with Cathy.
When she got home, Cathy told her father about her discovery. He returned he was doing what was good for her and she should do what he told her. Forbidden to visit Wuthering Heights out right, Cathy began to secretly write letters to Linton and delivered them with the help of a milkboy. Ellen found them and gave a choice to Cathy whether they should be burnt or be shown to Edgar. The first option was done.