Y.M.
This sounds like an "in the moment" kind of problem/solution situation.
I wouldn't prepare a speach or even set the situation up for a talk with your brother. I'd just take care of it in the moment.
But there are some things to consider before you approach the play time. The cousin may have trouble with over stimulation around other children, noise, activity... There are some activities you can introduce to the situation that have a focusing and calming effect.
Bring a bag of balloons. Most families avoid balloons because of the danger to babies, but if you supervise the activity and clean up, things should be safe. Have the kids blow up balloons and then sit on them to pop them by sitting on them. This activity provides focus in two ways. It takes focus to blow up the balloon and it requires a lot of energy to do it. Popping the balloon meets their physical needs to be rambunctious and jump about, but it takes the focus off of wresting each other. Just the blowing alone, can be calming and theraputic.... it may even calm them down.
The balloons is just an example of handling the situation "in the moment" and it doesn't require a big speach or parenting lessons. You'll just come across as a fun and nice Auntie, all of the kids will feel included, and your actions will demonstrate what you want - your actions will set the tone for the environment without having to say anything challenging.
It just really sounds to me like the little guy is struggling to socialize and calm himself down during play time. It may be a self regualtion issue also. Other calming activities are things like play dough... make some home made playdough and bring it over to the following play date.... Soon enough the kids will begin to expect a focused, calm, supervised, play time from you (the adult). Too much open activity might be too stressful for a kid that struggles with self regulation.
Let me know what you think.
Personal experience of late: Just the other day my son was playing with his good friend who has Asperger's (a mild form of Autism). The friend started punching him in the head. Rather than punishing the friend (who may never totally "get it") we switched gears and decided to take a walk to the park. That put all of that punching energy into the boys feet and wore him out. We walked up to the neighborhood park, met some puppies on the way, and the boys climbed and jumped. Our friends need for "joint impact" was met by jumping off of the slide at the park rather than punching my son in the head.
Look to the innocent... maybe the timeouts or even harsher punishment isn't the key here... maybe not the attitude, tone, or environment, anyone wants to spend time in. Kids do what the do to try to meet their needs and cry for help.
YM