Your younger cousin is copying your style, your words and your attitude. Give them something worth copying.

Copying happens below the level of admiration. A younger person picks up your phrasing, your posture and your attitude long before deciding whether they agree with you, and they take the unflattering parts along with the good ones. You are teaching whether or not you meant to.

Think about which of your habits you would be uncomfortable seeing repeated back at you, and change one. Leave the rest. Then give them something good on purpose, an interest or a standard they would not have found on their own.