If it's your dream job I'd still go for it though I would keep your options open. Especailly with the economy like it is right now.
Stay in, maybe find a way to file a complaint against your cousin for intentionally obstructing the process. You would need proof though.
Don't let your cousin make your decisions for you! Take the job if you really want and, somehow, 'survive' despite the a-hole. Remember, he's not the only one with power. You'll have it too. Deviously or otherwise, fight the guy.