John Wesley on Numbers 36:8

The inheritance of his fathers - This law was not general to forbid every woman to marry into another tribe, as may be reasonably concluded from the practice of so many patriarchs, kings, priests, and other holy men, who have married women of other tribes, yea sometimes of other nations, but restrained to heiresses, or such as were likely to be so. But if they had brethren, they were free to marry into any tribe, yet so that, if their brethren died, the inheritance went from them to the next a - kin of their father's tribe and family. And the principal reason why God was solicitous to preserve tribes and families unmixed was, that the tribe and family too, out of which the Messiah was to come, and by which he should be known, might be evident and unquestionable.