Aggressive behavior toward cousin?

He's just jealous and needs to be stopped or redirected. Preston is too young to know what the right thing to do is, so you need to show him. Mathew should not be hit, period. I would be upset too if no one stopped another kid from hitting mine. It's normal that it happens, it is not normal to allow it to continue. Hold his arms/hands and say gentle touches....then practice it and repeat. Be ready in situations that you know are triggers for him to hit. I know boys are rough, I have one. You can control him and it's your job!