Implied question: In Eruvin (21b), it says that when Shlomo enacted Netilas Yadayim and Eruvin, a Bas Kol said... Why doesn't it mention Sheniyos?

Answer: Perhaps he enacted them after the Bas Kol.

Citation (Yerushalmi): His father-in-law's wife is forbidden due to Mar'is ha'Ayin.

Citation (cont.) Suggestion: The Torah forbids her!

Citation (cont.) Rejection: David married Ritzpah Bas Ayah (Sha'ul's Pilegesh. The Gra (EH 26:7) derives that the Yerushalmi holds that Pilegesh is with Kidushin, so she is considered a wife. Some disagree; see Otzar ha'Poskim EH 26, Sa'if Katan 3.)

Citation (cont.): Two step-children who grew up together in one house are forbidden [to marry each other], due to Mar'is ha'Ayin;

Citation (cont.): R. Chanina says, they may marry in a place where people do not know [that they grew up together].

Episode: A case occurred in Province, in which the meat was slaughtered and the wine was mixed for a wedding feast for a man about to marry his father-in-law's [former] wife, and R. Tam [forbade the marriage and] spoiled his feast.

Implied question: Our Sugya permits the wife of one's father-in-law and step-son!

Answer: Perhaps afterwards, they forbade.

Observation: However, it is clear from Sotah 43b that we are not concerned for Mar'is ha'Ayin regarding a stepsister who grew up in the same house as her stepbrothers. (They may marry, for people know that they are only step-siblings.)

Implied question: Why don't we ask from the mother of the mother of his father-in-law? (Also she can say "I am permitted to you, but my daughter is not!")

Answer: This is a question below, whether there is a limit [to how many generations we forbid] regarding the Sheniyos of R. Chiya.

Question: Why don't we forbid her due to "any female who is Ervah, regarding a male, they decreed to forbid his wife like a Sheniyah", like we say below?

This is a real reason to forbid. It is not a mere Siman [when we forbade], for the Gemara asks "why are these [in which we forbid the wife of the corresponding male] different than these" [in which we do not forbid? We answer] "here he is related through [only] one Kidushin..."

Also, we resolve from it the wife of his mother's paternal brother. We do not establish it to allude to his daughter's daughter-in-law. (This shows that it is not a mere Siman.)

Answer (Ri): The Tana of the Beraisa does not hold like this (whenever a female is Ervah, we forbid the wife of the corresponding male), but later generations decreed it.

Support: This is why when we asked "what else did the Tana omit?", we did not want to answer that he omitted the wife of his mother's paternal brother, which we concluded is forbidden [since the Tana permits her. She was forbidden afterwards.]

Question: What was the question "is this a Klal (general rule, that whenever a female...)?! His mother-in-law is Ervah..."

Perhaps also these were forbidden afterwards, for this reason (the Klal)!

Answer: Rava knew that these are permitted [even nowadays].

Implied question: Below, we ask "according to you, there are 17"! (This is not difficult.) They decreed [to forbid the wife of his mother's paternal brother] afterwards, like I explained!

Answer: He knew that the Sheniyos of Mar brei d'Ravina were in the later generations [when they already decreed].

Explanation: Each one discussed one that was in his days.

Assertion: We must say that there were two with the name Mari bar Isak. One of them was in the days of Rav Ashi, like it connotes here and in Bava Metzi'a (22a);

Citation (22a): Ameimar, Rav Ashi and Mar Zutra visited the orchard of Rav Mari bar Isak.

There was another Mari bar Isak in the days of Rav Chisda, for it says (Bava Metzi'a 39a) "Mari bar Isak, and some say Chanah bar Isak, a brother came to him from Bei Chuzai. He came in front of Rav Chisda."

Rav Chisda died in the days of Rava, like it says in Yoma (75b), and on the day Rava died, Rav Ashi was born, like it says in Kidushin (72b).

Observation: We could have asked about the sister of the mother's mother, who is included in the sister of the father's father, for below her is Ervah.

Alternate text: In some texts, the Gemara explicitly asks this.

Explanation: Even though these are two, they are counted like one, like we say below "these two resemble each other, so they are counted like one";

Explanation #1 (Rashi): [There,] the two are the paternal grandfather's sister and the paternal grandfather's brother's wife.

Explanation #2 (Rivam): They are the wives of the mother's maternal and paternal brothers, for all of them are called the mother's brother.

Opinion #1: R. Chananel rules like Ameimar, and so does the She'altos of Rav Achai.

Support: Rav Ashi holds like this, for he said "even though you saw it written that they are forbidden, [do not rely on it, since] Mar brei d'Ravna didn't sign on it!"

Opinion #2 (Ri): The Halachah does not follow Ameimar, for the Sugya below (40b) is unlike him. (It asked whether Chachamim decreed Sheniyos regarding a Chalutzah);

The Gemara says that Ameimar must say that the Isur (of a Chalutzah to the grandson of the Choletz) is due to the Choletz (but not because she was once married to his grandfather's brother). He holds that we decreed Sheniyos regarding a Chalutzah. The Gemara did not want to settle the question from him.