The Tosfos Rid here writes that there is a difference between "Kurvah" and between "Achvah".

1) The Torah forbade the father's wife because she is a Kurvah of his father; she is close to him. Since the Torah forbade his father's Kurvah; namely his wife; Chazal came along and made a Gezerah even on thefather's father's wife.

2) The prohibition of Achvah is lighter. Brotherhood is further away so Chazal were not so machmir to make Gezeros on this. [We can say that a man is closer to his wife than to his sister - DB]. This is why Chazal did not make Shenios on Achvah; to make prohibitions on generations above his sister or generations below his sister, so one is allowed to marry the mother of one's sister-in-law. For Kurvah they were more machmir; for instance one may not marry one's father's mother or grandmother.

3) Ispaleg Dorosa means that when only Achva; brotherhood; is involved if it is one generation further away Chazal did not make a gezera so one may marry one's grandfather's sister according to Ameimar. In contrast Chazal were less lenient for Kurvah which is why one may not marry one's grandfather's wife.

Good Shabbos Hagodol!

Dovid Bloom

Follow-up reply:

1) Tosfos Rid writes that the reason for the difference between Kurvah and Achvah is because Kurvah extends up to earlier generations, and down to later generations and therefore it all looks like one house so Chazal made a Gezera on all relationships, both higher up the family tree, and also lower down. In contrast, for those relations which are forbidden because of Achvah, for instance the sister or sister-in-law, Chazal did not make Gezeras up or down the generations. This is because Achvah is like a different house, so if one permits the sister-in-law's daughter, for instance, people will not confuse the sister-in-law's daughter with the sister-in-law herself because everyone understands that one has not permitted a marriage inside the same house. Therefore Ispaleg Doroso is sufficient to make a gap between the relationships of Achvah.

[Possibly we can understand this by saying that a person tends to think that he is part of his grandfather's family but he does consider himself as part of his great-uncle's family. This is because his grandfather is head of one household, and his grandfather's brother is the head of a different household. Achvah does not make it the same household - DB].

2) Rashash here also asks why the grandfather's wife is prohibited, even though she would appear to be ispaleg? He cites Tosfos above 3a DH Mekamei who writes that a daughter-in-law is a closer relative than a sister-in-law. This is because "Bra Kra'eih deAvuha" (see Eruvin top 70b). The son is the leg of the father. A son is merely a continuation of his father. A man is more closely realted to his daughter and grandaughter than he is to his sister. Tosfos 3a seems to be quite similar to the Tosfos Rid in explaining the difference between the "vertical" extension of the family tree and the "horizontal" extension.

3) Chidushei Rabeinu Meir Simchah here (by the author of Or Sameach and Meshech Chochmah) gives an alternative explanation of the difference between the grandfather's wife and the grandfather's sister. This is because relations with one's father's wife carry a capital penalty (see Rashi Yevamos 49a DH Lo Yikach) whilst relations with one's father's sister carry the lighter punishment of kores (see Mishnah Makos 13a). Therefore when Chazal came to consider whether or not to make a Gezera for one generation up, they were stringent on the grandfather's wife and lenient on his sister.

[There seems to be a small printing error in the words of Chidushei Rabeinu Meir Simchah, but I have explained it according to my understanding of what he means]

KOL TUV

Dovid Bloom