Only to the family of the tribe of their father shall they marry. The direction is not altogether plain, since the tribe (matteh) contained several families (mishpachoth), and in this case one or more of the families were widely separated from the rest.
Probably the words are to be read, "only to the tribe-family of their father," i.e; only into that mishpachah of Manasseh to which their father had belonged. Practically, therefore, they were restricted to the family of the Hepherites (Numbers 26:32, Numbers 26:33).
This is made almost certain when we remember that the territory of the "family" was to be apportioned within the tribe in the same way, and with the same regard to relationship, as the territory of the tribe within the nation (see on Numbers 33:54).