Lock your doors and hide your car!
At least she cannot get in. If she calls later to ask where you were just make something up. If you are 'out' more often when she comes over Im sure her visits will cut back bc she won't expect you to just be home when she feels like coming over. Thats a tough situation. My Best friend has a 4 yo boy who is out of control, he kicks my dogs. I have to put my dogs in my room when she brings him over and tell him they are tired and need a nap. She does nothing while hes trying to kick the puppies and saying out loud "KICK the PUPPIES!" I have to tell him to stop. He just does not listen at all. When she discplines him, she SCREAMS and it sounds like 2, 4 year olds going at it. Its gotten to the point where I have not seen her in over 3 months now. I just cannot take her kid at all anymore. He over stimulates me and she does too. Maybe you can tell your cousin that you just cannot take the rowdiness of the kids when you are trying to work. Make sure she knows that even if you are home, you are 'at work' technically and she can't come barging in during the day like she does. Just say it nicely. Im sure you won't get family issues going if you are polite and nice.
Im sorry though, that does stink!