Since both accounts (yours and your brother's) were accessing the site from the same IP address, and because one account (your brother's) had upvoted posts made by the other account (yours), a moderator (or an automated script) undoubtedly concluded that you were engaging in vote fraud.
Typically, a user will register a second "sockpuppet" account and use that to upvote their own questions and answers posted from their main account, earning reputation along the way. This is considered a massive abuse of the system and is strictly forbidden. That's precisely what you were doing by having two accounts registered from the same IP address that were voting on each other's content.
Since the second account (your brother's) was deemed to be a sockpuppet, it was merged into the main account (yours). That's why when your brother logs in, he sees your account. There are no longer two separate accounts.
As well, the single account that remained (yours) was suspended as a result of this abuse. The suspension message displayed on your profile is the evidence of that:
This account is temporarily suspended for voting irregularities. The suspension period ends in 2 days.
I suppose that if you really do have a brother who created a separate account, but you share the same computer/Internet connection, it's possible that the ban was levied in error. You could probably flag one of the moderators on that site and ask them to look into it.
But you should tell your brother not to make a habit of voting for your posts. That's not really fair. I don't have a brother—who's going to vote for my posts? Besides, we have no way of verifying that it is, in fact, your brother.