The system will not let you register if there is already an account registered on your IP address. Try registering using a proxy, and then you should be able to access the site after that.
Also, be aware that you are responsible for your own IP address. Your brother is not banned and so you are both allowed to use the site, but if one of you becomes banned, you will BOTH be banned from using the site since you share an IP address.
Please read the rules so you are also aware of our rules regarding account interaction.