Numbers 36:6

They seem hereby to be confined not only to the same tribe, trot also to the family of their tribe, as appears from the reason of the law, for God would have the inheritance of families as well as tribes kept entire and unmixed; and this was one reason of that law of marrying the brother's wife, Deu 25. And although the next verse may seem to thwart this interpretation, the reason of this law being there given that inheritances might not go from tribe to tribe, Numbers 36:8 confirms it, where the very same phrase is repeated, and that more emphatically, that such shall marry one of the family of the tribe of her father; and this further reason and restriction is added, that they may enjoy every man the inheritance (not only of his tribe, but) of his fathers, to wit, belonging to his father's family.