I tend to have dreams about my cousin pretty often.
What is your dream including your cousin like?
I'm taking a revenge on him because he makes me feel so pissed off
You need to remember a dream is just a dream. Do not take what happened in the dream into reality.
I'm going to hurt him in real life.
Just because you had a dream about you doing something, does not mean that it is reasonable to do in real life. The main message in your dream is simply that you feel hurt by something your cousin did. What you should instead do is talk to your cousin and let him know how he made you feel.