You cannot claim someone as a dependent and use their personal exemption on your tax return if the other person is also filing a return claiming their own personal exemption.
If your brother is age 19 or older and not a full time student, you would only be able to claim him as a dependent if he met the requirements under the Qualifying Relative rules. If he has gross income of $4,000 or more for the 2015 tax year, you cannot claim him as a dependent.
Qualifying Relative -
1. The person cannot be your qualifying child or the qualifying child of any other taxpayer. A child is not the qualifying child of any other taxpayer if the child's parent (or any other person for whom the child is defined as a qualifying child) is not required to file an income tax return or files an income tax return only to get a refund on income tax withheld.
2. The person either (a) must be related to you or (b) must live with you all year as a member of your household.
3. The person's gross income for the year must be less than $4,000 (social security does not count) in 2015
4. You must provide more than half of the person's total support for the year.
5. The person must be a U.S. citizen or a U.S., Canada, or Mexico resident for some part of the year.
6. The person must not file a joint return with their spouse.