You're Catherine's sister. You grew up with her and Heathcliff. She was a spoiled child, always talking about dresses, money, and how she was going to marry a rich man. You and Heathcliff laughed at her for it; you knew she dreamed too much. You used to spend all day together, until you grew up and she became focused on going to town with Nelly to find someone she liked. This meant you spent more time with Heathcliff, and because there was less money, you started helping your father and tending to him when he came home drunk, which led to many arguments between you and Heathcliff.
He never liked how your father treated you or Cathy. That's why, from childhood, he defended you both and endured your father's beatings and punishments. So, the fact that you were so attentive to him despite his awful attitude made Heathcliff angry with you.
Today wasn't much different. You helped your father eat after he came home drunk. Heathcliff saw you and refused to speak to you all day. So, at nightfall, you decided to go to his room, which was nothing more than a space upstairs in the horse-raising hut.
You climbed the ladder and approached the curtain that separated you from him until you heard noises downstairs, so you lay down on the floor to see through the small gaps in the woodwork. You saw Joseph enter with a girl who helped in the house, laughing and smiling. You were curious to know why they were there. Until you saw them kissing... and undressing... and using the horse muzzles for... their activities. You had never seen anything like it; you didn't know it existed. You lay there stunned, watching them, until you felt a hand over your mouth and another covering your eyes.
You lay still, listening to the moaning from downstairs, feeling a strong chest pressed against your back. You knew who he was. And after the two lovers downstairs finished their... things, you felt both hands covering you move away and slowly turned on the floor to look at Heathcliff, who lay beside you, so close you could feel an erection against your backside. You stared at him, confused, not quite understanding what had happened.
"Shh." Heathcliff reached out to brush a strand of hair away from your face as they waited for the other two to leave.