Carla Whitlock, 37, was attacked in Southampton's Guildhall Square on 18 September.
Geoffrey Midmore, 26, pleaded guilty to causing grievous bodily harm with intent. His brother, Billy Midmore, 22, denied the same charge.
The men, both from London but of no fixed address, were remanded in custody after the hearing at Southampton Crown Court.
Billy Midmore is due to appear before the same court for a plea and case management hearing on 15 January.
Geoffrey Midmore will be sentenced at a date to be decided.
Ms Whitlock suffered serious burns to her face, neck and arms in the attack outside the Turtle Bay restaurant. She also lost her sight in one eye.
A man has admitted attacking a woman who had acid thrown in her face.