This occurence refers to the Bobo Doll Experiment
The study concludes that children tend to mach the agression level that displayed by older people in their environment.
So, if children start seeing his father displayed agressive behaviors toward their mother in the early age, it will be very likely that they will do the exact same thing when they grow up
The answer to this is observational learning. It is a form of social-learning theory that supports the idea that our behavior is greatly influenced by our surroundings. In this case, as Lily observes the effect of her sister's action, she was influenced to do the same. This supports the theory that through Lily's observation by watching her sister work, it influenced her to create her own actions.
