If you're in the same room and therefore on the same local network you shouldn't need to change anything in the router. Just try to connect from your PC to the laptop's local IP address (the one you get from within Windows, not your WAN IP).
That said, not really sure what you are trying to achieve here, Windows 8.1 will only allow one user to be using the machine at a time, even when one is connecting over remote desktop - If you try to log in it will lock your brother's session on the laptop.