Big sister Rui returned to her parents' home after fighting with her cohabiting boyfriend. She planned to stay in her little sister's room for a while, but her sister has a lovey-dovey boyfriend and treats her as an nuisance. One day, she accidentally hears the two having sex and starts masturbating to their kissing and moaning sounds. Her pussy gets soaking wet, and unable to hold back, even though she knows it's her sister's man, her greedy personality gets in the way and she reaches for his cock…