Several authorities write that where no Levites are present, a firstborn (from his mother) should wash the hands of the Kohanim (Bach in the name of Mahari Molin; Taz 128:4; Magen Avraham 128:7).
The Kaf HaChaim cites this halachah in the name of the Ben Ish Chai, yet concludes: “However, I have not seen anybody practicing this custom, and it appears that they rely on the simple reading of the Zohar, which implies that only a Levite washes the Kohen’s hands. If there is no Levite, the Kohen should wash his own hands.” Today, the washing of Kohanim’s hands by firstborns is virtually unheard of.
Poskim add that an Israelite should not wash the Kohanim’s hands, and where there are no Levites or firstborns, a Kohen should wash his own hands. Shut Nishal Le-David (91) writes at length on the question of whether one Kohen can wash the hands of a fellow Kohen, but the widespread custom is that a Kohen washes his own hands.
