Yes you can do that, some servers only allow
one account per IP address.
They will allow multiple lines to run off the account
which don't have to be on your IP.
So you have two options;
Take your laptop to your brothers , set up the server
account details there ( using his IP )
Then put the line in the box and it should be fine.
The only problem is he will need to get on the net at some point to extend / renew the line.
Or, the other option is you just order another line at your house, (test it in your
box if you want), then just put the line in his box and you extend / renew it for him.