The Gemara has a rule, אין מזהירין ואין עונשין מן הדין - we do not warn (source a negative commandment) or punish (source for the punishment of a negative commandment) from the logic of a קל וחומר. The reason for this is debated, either because any קל וחומר can be somehow disproven, or because of the way punishments work, or because of a גזירת הכתוב (i.e., that's just the rule). But the reasoning for this is not what we are discussing.

The source for this rule can be found in Makkos 5b. The Torah prohibits marriage to both your half sister from your mother, and your half sister from your father. One could make the following קל וחומר - if a half sister is prohibited, shouldn't a full sister be prohibited? However, we learn the prohibition for a full sister from a drasha rather than this קל וחומר, proving that we never use קל וחומר's to learn prohibitions. (See the Gemara for more detail.)

The question is the following: elsewhere (eg Kiddushin 4a), the Gemara says that "מילתא דאתיא מק׳׳ו טרח וכתב לה קרא" - something learned from קל וחומר, the Torah sometimes goes and says anyway, even though I would have logically figured it out on my own. Doesn't such a rule disprove the proof about אין עונשין מן הדין given in Makkos 5b?

  • I was literally just learning Keritot last night. Look at 3a and I think a potential answer to your question is that not everyone holds אין עונשין מן הדין- in the Gemara over there I think it was Rabbi Yitzchak who doesn’t hold this. So according to him the other Kiddushin statement makes sense. Also note that it’s not so clear that the sister pasuk is the source for this אין עונשין. I think the Rosh only suggests this but I think there’s a different source Commented May 10 at 20:05

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Perhaps a full sibling isn't required to be derived at all as all whole siblings are also half siblings.

Maybe "kal vachomer" (or "din") is being used in a borrowed and dramatic manner.

If the Torah still goes out of its way to record a whole sibling it can only be to teach us that אין מזהירין/עונשין מין הדין.

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