Albert Barnes Commentary Nehemiah 6:18

"For there were many in Judah sworn unto him, because he was the son-in-law of Shecaniah the son of Arah; and his son Jehohanan had taken the daughter of Meshullam the son of Berechiah to wife." — Nehemiah 6:18 (ASV)

Though Tobiah is called the servant or slave (Nehemiah 2:10, 19), and was perhaps a bought slave of Sanballat’s, yet he was in such a position that Jewish nobles readily contracted affinity with him. This is quite in harmony with the practice of the East, where slaves often fill high positions and make great marriages.