Resources for Yevamos daf 8
1. The גמרא tells us that הואיל ואישתרי אישתרי only works if the היתר occurred before the איסור came, but if she was an אחות אשה before she became an אשת אח then we would not say הואיל ואישתרי אישתרי. The קובץ הערות in סימן י"א אות ח says that there are two ways to understand הואיל ואישתרי אישתרי in regards to our גמרא. One way of understanding it is that once the איסור is חל then the היתר doesn’t have the ability to uproot it. However, another way to understand our גמרא is that if the איסור came first then instead of saying הואיל ואישתרי אישתרי we should say והואיל ואיתסר איתסר! The נפקא מינה would be if the איסורים occurred at the same time. If the פּשט in our גמרא is that once an איסור is חל we can’t uproot it, then if they occurred at the same time the איסור wasn’t חל yet and we could still say הואיל ואישתרי אישתרי. However, if the פּשט in our גמרא is that we can say הואיל ואיתסר איתסר just as easily as הואיל ואישתרי אישתרי , then if the איסורים would occur at the same time we wouldn’t know which way to turn and you would need to be מחמיר. He leaves it a צריך עיון.
2. There is a חקירה in all of מסכת יבמות regarding the איסור צרה: is the איסור צרה a new type of איסור ערוה, similar to אחות אשה, that happens to only exists by יבום where the צרת ערוה becomes like the ערוה itself? Or do we say that לצרור just tells us that the צרה is פּטורה from יבום and ממילא the original איסור אשת אח comes back. רש"י on דף ג in ד"ה משו"ה and in other places seems to say that ערוה merely exempts a צרה from יבום and ממילא she has איסור אשת אח. However, the חידושי מרן ריּז הלוי in הלכות יבום וחליצה and the משנת רבי אהרן in סימן ב אות ב say that our גמרא seems to not be going like that. One clear proof is that the גמרא says that we need a פּסוק to teach us that a צרה is not אסורה in a case of שלא במקום יבום. If צרה was just a פּטור יבום then that line would seemingly make no sense as שלא במקום מצוה there is no אשת אח at all so how could she be אסורה! However, the אחיעזר in סימן א אות ט"ו says that although our גמרא does say that, our גמרא is only in the הוה אמינא stage, but once we know that a צרה is מותרת שלא במקום מצוה, then we change to the idea that צרה is just a פּטור.
3. The גמרא brings a פּסוק to say that a יבם can divorce his יבמה after יבום and can even remarry her afterwards, but it doesn’t need a פּסוק to prove that after ביאה ראשונה she is allowed to stay married to her יבם even though the מצוה of יבום would seem to have been completed. תוספות in ד"ה מלמד asks why this is this true? After all, we say that חייבי לאוין can’t do a ביאה שניה since the מצוה was already completed, so why don’t we need a פּסוק to say that a יבם can do a ביאה שניה? He answers that it is a סברא that the תורה didn’t give a מצות יבום with the intention that you divorce her after the ביאה ראשונה. The מרומי שדה has a different answer which is consistent with his שיטה in all of ש"ס. He answers that מצוות have minimum שיעורים but if you do more than the minimum it is still considered the מצוה. For example, in חגיגה דף ח ע"ב it says that if you decide to bring ten קרבן חגיגהs (nine more than necessary), they all count as a קרבן חגיגה and תוספות says from the ירושלמי that they are all דוחה יו"ט. You see from there that doing more מצוה than the תורה requires is still called a חלק of the מצוה. The same is true for eating more than a כזית of מצה on פּסח. The entire אכילה is considered a חפצא של מצוה. Similarly, as long as the יבם and יבמה are married the מצות יבום continues. However, that is only true where it is הותרה. However, when it comes to דחיה then you can only do the minimum amount. Therefore, by regular יבום they can stay married forever as it is הותרה, whereas by חייבי לאוין it is דחויה so you can only do ביאה ראשונה.
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