Daf Yomi Yevamot 17a: Rav's Derashot About אשת אחיו שלא היה בעולמו and אחין מן האם as Peshat

On Yevamot daf 17a, Rav gives derivations for two halachot via derasha, but the derashot actually strikes me as true on a peshat level as well, as coming from those particular words or thereabout.

Citing my translation of the Rif:

{Yevamot 17a}
Gemara:

אשת אחיו שלא היה בעולמו היכא כתיבא
אמר רב יהודה אמר רב אמר קרא כי ישבו אחים אחים שהיתה להם ישיבה אחת בעולם פרט לאשת אחיו שלא היה בעולמו
יחדיו המיוחדין בנחלה פרט לאחין מן האם:

The wife of the brother who was not in his world, where is it written?
Rav Yehuda cited Rav: The verse stated {Devarim 25:5}

ה כִּי-יֵשְׁבוּ אַחִים יַחְדָּו, וּמֵת אַחַד מֵהֶם וּבֵן אֵין-לוֹ--לֹא-תִהְיֶה אֵשֶׁת-הַמֵּת הַחוּצָה, לְאִישׁ זָר: יְבָמָהּ יָבֹא עָלֶיהָ, וּלְקָחָהּ לוֹ לְאִשָּׁה וְיִבְּמָהּ.

5 If brethren dwell together, and one of them die, and have no child, the wife of the dead shall not be married abroad unto one not of his kin; her husband's brother shall go in unto her, and take her to him to wife, and perform the duty of a husband's brother unto her.

Brothers who have a single {shared} dwelling in the world, to the exclusion of the wife of the brother who was not in his world. {The next word:} יַחְדָּו - together - who are distinct in inheritance, to the exclusion of brothers from the mother.

This all sounds very midrashic, but in fact these deductions make good sense as peshat within the general parsha, and even good peshat on the words from which they are derived as well.

The pasuk states כִּי-יֵשְׁבוּ אַחִים יַחְדָּו, if two brothers dwell together. Let us step back a moment and think how it is that two brothers dwell together. In Biblical days past, there was an extended family unit headed by the paterfamilias, which we might call the bet av. Brothers, with their families, dwelled together. The idea seems to be that when a woman is widowed without children, she is not abandoned and alone in the world, but another brother steps up to the plate and takes her into his household, and furthermore that they continue that particular branch of the family. There is an element of duty to the brother as well, and also inheritance. As we read (same perek):

ו וְהָיָה, הַבְּכוֹר אֲשֶׁר תֵּלֵד--יָקוּם, עַל-שֵׁם אָחִיו הַמֵּת; וְלֹא-יִמָּחֶה שְׁמוֹ, מִיִּשְׂרָאֵל.

6 And it shall be, that the first-born that she beareth shall succeed in the name of his brother that is dead, that his name be not blotted out of Israel.

shem means inheritance, or as I like to translate it to English, "title." We see this explicitly by Ephraim and Menashe, or actually children after them, about whom we read {Bereishit 48}:

ו וּמוֹלַדְתְּךָ אֲשֶׁר-הוֹלַדְתָּ אַחֲרֵיהֶם, לְךָ יִהְיוּ; עַל שֵׁם אֲחֵיהֶם יִקָּרְאוּ, בְּנַחֲלָתָם.

6 And thy issue, that thou begettest after them, shall be thine; they shall be called after the name of their brethren in their inheritance.

and if further evidence is necessary, the yibbum by Rut is lehakim shem haMet al nachalato, and the child born is not named Machlon or Kilyon.

Considering all this -- when are the two brothers dwelling side by side under the same paterfamilias? When both are alive at the same time. (In fact, he does not, but we may stress yachdav.) If so, a brother born after the death of his brother does not dwell with him, and would not be expected to assume these responsibilities. Indeed, before he was born, she could have gone off to marry some stranger (or some other brother). And where there is no obligation of yibbum, there is a prohibition of eshet ach, the brother's wife, just as she is prohibited to him had she had children.

In terms of a maternal brother, such a brother would be in a different bet av, with a different paterfamilias. They would not be yachdav, or dwelling yachdav. And at issue is inheritance, (as indeed they darshen yachdav as מיוחדין בנחלה) and what business does a maternal brother have to inheritance from someone who is not his father?

Next up, how sefer Rut challenges these two laws.