One of the descendants of Jethro was Jonadab, son of Rechab, who, when he heard from a prophet that God would destroy the Temple, bade all his children, as a toke of mourning, to drink no wine, use no oil for anointing themselves, nor cut their hair, nor dwell in houses. The Rechabites obeyed this command of their sire, and as a reward for this, God made a covenant with them that their descendants should always be members of the Sanhedrin, and teachers of Israel.
The covenant with the Rechabites was even stronger than that with David, for to the house of the latter God promised to keep the covenant only if his descendants were pious, but He made an unconditional covenant with the Rechabites. God rewarded them for their devotion to Him in this way, although they did not belong to the Jewish nation. From this one can gather how great would have been their reward if they had been Israelites.