There is no fool proof method using a router to prevent an authorized user from downloading data in excess of an arbitrary quota. You can slow someone down or stop their Internet access, but even with whatever restrictions you put in place a knowledgeable user can easily get around them by changing their IP and or MAC address.
I would instead suggest you get a router that monitors and logs data downloaded, have a talk with your brother and tell him point blank you are monitoring usage and if exceeds your CAP he will be responsible for paying the overages.
If he doesn't agree or fails to pay then change the SSID on your router as well as physically locking the router up so he can't access an Ethernet port or the router' reset button.
Two hundred GIGs is a very generous download CAP that most families would not even come close to utilizing even with lots of video streaming in HD. IMHO to exceed your CAP your brother would be downloading DVD rips, which if it is the case and the copyright holders file a complaint with your ISP. Several complaints could result in your Internet service being suspended. Just another reason to check what your brother is doing.
200GB is indeed too limited having 4 people in my family using internet daily. My brother loves to test out free games on Steam for an hour and then deletes them. He also recently bought a PS4 and one game download adds up to 35-40GB download. Unfortunately I'm stuck with my ISP as there is no better alternative around here.