A grandson of Eliashib the high priest became son-in-law to which enemy of Nehemiah?

Answer: Sanballat the Horonite

One of the sons of Joiada son of Eliashib the high priest had become Sanballat's son-in-law, so Nehemiah drove him away.

“Even one of the sons of Jehoiada son of Eliashib the high priest had become a son-in-law to Sanballat the Horonite. Therefore I drove him away from me.”— Nehemiah 13:28 (BSB)