Many routers you can default all all mac addresses and then block just certain ones. Still you would have to know those mac addresses.
Now if they are not to ever use the internet it should be as simple as not telling them the password.
There are many more complex solution but your router would have to have the ability. Some have a guest wireless that you can restrict by time of day. The only true way to control access is to use enterprise wireless mode but that requires you have a radius server of some kind. When you use enterprise mode every person has a unique id and password and you can set access rules in the radius server.
Many routers you can default all all mac addresses and then block just certain ones. Still you would have to know those mac addresses.
Now if they are not to ever use the internet it should be as simple as not telling them the password.
There are many more complex solution but your router would have to have the ability. Some have a guest wireless that you can restrict by time of day. The only true way to control access is to use enterprise wireless mode but that requires you have a radius server of some kind. When you use enterprise mode every person has a unique id and password and you can set access rules in the radius server.
You will know MAC addresses of your cousins once they arrive, so this is when you can enter them in your router. Not the most pleasant task, but unless you have a boatload of cousins, you can do that.
Read carefully and only change one thing at a time. Keep notes on you do so you can reverse or otherwise change some setting that does not work as expected.