"Do you have a brother?"
Translation:¿Tú tienes un hermano?
Why not "¿Tienes tú un hermano?" is promoted as answer? Is not it the one that is grammatically correct? It is accepted anyway.
You can use 'tiene' as long as you use 'usted' as well to make it a formal question, e.g. ¿Tiene Ud. un hermano? 'Usted' usually shortens to 'Ud.' (with capital letter and full stop) in written Spanish.
¿Tienes usted un hermano? I would use to say do you have a brother... Tu tienes un hermano is more of a question response to someone finding out that the subject has a brother