Numbers 36:8

Women. Hebrew, "every daughter that possesseth an inheritance....shall be wife to one of the family of the tribe of her father." Commonly the females were debarred from inheriting land, when they had any brothers. The Levites were not concerned in these regulations, as they had no inheritance; and hence, we need not be surprised to find that St. Elizabeth, of the daughters of Aaron, (Luke i. 26,) was related to the blessed Virgin, who was of the family of David. The mother of St. Elizabeth might be of the tribe of Juda; or a maternal ancestor of the blessed Virgin might spring from the tribe of Levi. (Calmet) --- Tradition determined the lawfulness of such marriages, and in this case, St. Augustine (Consens. Ev. ii. 23) admires the providence of God, in causing his beloved Son, the great anointed, to be born both of the regal and priestly tribes, in which an unction was required, before the priests and kings were put in possession of their respective offices. Thus Christ was both priest and king, and such were anointed in the law of Moses. (Worthington)