Matthew Poole and continuators on Numbers 36:8

8 And every daughter that possesseth an inheritance in any tribe of the children of Israel, shall be wife unto one of the family of the tribe of her father: that the children of Israel may enjoy every man the inheritance of his fathers.

By which clause it seems this Law was not general to forbid every woman to marry into another Tribe (as may be reasonably concluded from the practise of so many Patriarchs, Kings, Priests, and other holy men, who have married women of other Tribes, yea sometimes of other Nations, which it is not likely they would have done, if this had been a transgression of Gods Law) but restrained to heiresses, or such as were likely to be so. See 1 Chron. 23. 22. But if they had brethren, it is probable they were free to marry into any Tribe, yet so that, if their brethren dyed, their punishment was, that the inheritance went from them to the next a kin of their fathers 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.