Yes, you may be able to buy life insurance on a sibling.
However, the insurance company may or may not require your brother's signature on the application for life insurance.
Brothers may have insurable interest in one another, which is required in order to buy life insurance on someone.
It means, you either rely on your brother for financial support, or that you brother's death would cause you a financial cost; such as, having to pay for his burial expenses.
Learn more about getting life insurance on your brother.