They might see your two accounts as multi accounts on their site because you two are connecting to it using the same internet connection I am guessing since you say you two are brothers and usually they live together in the same residence.
Yes
Just wait until you receive an official response from them into why they banned your account if you did nothing wrong their site.
It is strange they do not allow you to participate in the bctalk forums contests if they are not thinking you are using multiple accounts on their site.