Answer:
Tu
Step-by-step explanation:
Tu should be used in more informal situations. For example, when speaking to a family member, spouse, or classmate.
Vous is used in situations that are more formal, like if you are talking to people you don't know, a boss, a teacher, or elders.
Since you are speaking to a cousin, they are a relative. Therefore, tu is the best pronoun for this circumstance.