29 Added to this; there is another way peculiar to the Jews, in which a man might be the son of another of whom he was not born according to the flesh. For kinsmen used to marry the wives of their next of kin, who died without children, to raise up seed to him that was deceased. So then he who was thus born was both his son of whom he was born, and his in whose line of succession he was born. All this has been said, lest any one, thinking it impossible for two fathers to be mentioned properly for one man, should imagine that either of the Evangelists who have narrated the generations of the Lord are to be, by an impious calumny, charged so to say with a lie; especially when we may see that we are warned against this by their very words.
For Matthew, who is understood to make mention of that father of whom Joseph was born, enumerates the generations thus: “This one begot the other,” so as to come to what he says at the end, “Jacob begot Joseph.” But Luke— because he cannot properly be said to be begotten who is made a child either by adoption, or who is born in the succession of the deceased, of her who was his wife— did not say, “Heli begot Joseph,” or “Joseph whom Heli begot,” but “Who was the son of Heli,” whether by adoption, or as being born of the next of kin in the succession of one deceased.
Source: Sermons on Selected Lessons of the New Testament (New Advent)