Rashi on Numbers
וגם את אחיך AND ALSO THY BRETHREN — i.e. the sons of Gershon and the sons of Merari.
Sforno on Numbers
וישרתוך, seeing their service is meant exclusively to assist the priests in their tasks.
Siftei Chakhamim
In also warning… Not that they should be joined in the warning that you were told about — warning the Levites about the service of the kehunah. Because the service of the kehunah was not given over to them, so how could they warn the Levites about it? Rather, just as a kohein was commanded to warn the Levite not to come into contact with the service of the kehunah, so too the Levite was commanded to warn the Israelite not to come into contact with the service of the Levites. Both would bear their sin if they did not warn them.