Is the husband sinful for preventing his sister from completing her marriage and insisting on dissolving the engagement, despite her desire and that of the groom's family, based on a dispute over the mother's place of residence after travel?

As long as the man has contracted marriage with your sister-in-law, neither her brother nor anyone else has the right to prevent her from being handed over to him. If your husband has prevented his sister from traveling to her husband for no valid reason, but merely due to formalities, customs, and traditions, then this is impermissible, a forbidden injustice, and a grave sin. He must repent to Allah and not stand between her and her husband. Your sister-in-law should travel to her husband, and she is permitted to refer her case to the judiciary if her family prevents her from traveling.