Final answer:
The sister flinches when the boy raises his hands because he is throwing mock punches at her. Mock punches are fake or pretend punches, and the sister's flinching is a natural reaction to protect herself.
Step-by-step explanation:
The sister flinches when the boy raises his hands because the boy is throwing mock punches at her. Mock punches are fake or pretend punches, meant to simulate a real punch without actually making contact. The sister's flinching is a natural reaction to protect herself, even though the punches are not real. This situation shows the importance of understanding non-verbal cues and body language in interpersonal interactions.