I like Rosecity's answer, since you already have a history of these kids coming over 10-20 times a day. Take the approach that you hope the noise level isn't irritating to them, and you might add that you will try to control the parking in front of their house, but they should let you know if any of your FAMILY MEMBERS park them in. You might comment that you are worried about getting everything done as well as the supervisory aspect with so many people but you are glad YOUR FAMILY can have this unique opportunity for cousins to get together.
I think sending them home if they just show up with no warning could be difficult and could even provoke the poorly behaved child. You can say to them, "I'm so sorry, boys, but this is just for cousins who don't get to see each other very often, and we will see you another time." If that doesn't work, have a designated person in your family (your husband?) to take the kids home and say to the parents, "I'm sorry, but we just can't supervise extra children today." If he has to, he can comment that "as you know, Johnny requires extra supervision anyway, and we simply can't provide that level of oversight in this crowd. Besides, it's all family and we didn't allow anyone else to bring friends from outside the family."
I don't agree with a sign on the door if it sounds exclusionary like "family only" but I think it would be fine to highlight your location with a "Smith Family Reunion" sign in the yard as if you are making it easy for those who are coming from out of town, rather than keeping out the neighbors.
The fact that the bully child only picks on smaller kids isn't the least bit surprising. He's aggressive but not stupid. My heart goes out to him but all you can do is protect your own children, short of calling child protective services to report that he's a menace to his brother or is being neglected by his parents.