Yes, it is possible the brother has legal standing to sue in your situation. The scenario is really no different from a property management company suing for unlawful detainer on behalf of the owner. You really have to look to see how the complaint for unlawful detainer alleges the plaintiff's status and role.
Frank W. Chen has been licensed to practice law in California since 1988. The information presented here is general in nature and is not intended, nor should be construed, as legal advice for a particular case. This Avvo.com posting does not create any attorney-client relationship with the author. For specific advice about your particular situation, please consult with your own attorney.
In addition to what Mr. Chen said, in an unlawful detainer proceeding you generally cannot challenge the right of the plaintiff from whom you rented the premises to bring the action. If the brother could not have rented it to you, then you have no right to be there.