According to Mark 6, Herodias had been the wife of which brother of Herod?

Answer: Philip

Herod married Herodias, his brother Philip's wife, and John told him, "It is not lawful for you to have your brother's wife."

“For Herod himself had ordered that John be arrested and bound and imprisoned, on account of his brother Philip’s wife Herodias, whom Herod had married. For John had been telling Herod, “It is not lawful for you to have your brother’s wife!””— Mark 6:17-18 (BSB)