Shaale on daf 21
Why is a wife's grandmother, and a wifes grandaughter, and your own granddaughter all ossur midoiraisa. But a blood grandmother is only a sheniyah?
Why is the kurva of a wife's grandmother worse than his own?
Avrumi Hersh, London england
Sholom Rav.
In answer to your question, the Me'iri writes that a person has a stronger tendency to sin with his mother-in-law than with his grandmother. (Presumably, he is referring to a physical attraction.)
On the other hand, he writes, the Torah forbids a mother only because when one is small one is constantly together with her, and is even used to being held by her flesh to flesh.
Be'Virchas Kol Tuv.
Eliezer Chrysler
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