My question is, Rashi says the Shniya loses her Ksuba because presumably she encouraged the marriage since neither she nor her children become Psulim from this marriage. I understand then why the Grusha l'Kohen gets a Ksuba--because although she is already Psula, her children will become psulim from this marriage (Challalim) and therefore we would assume that the husband encouraged the marriage. However, I don't understand the Halachah of the Mamzeres/Nesinah l'isroel. She and any future children are already Psulim to begin with--therefore she is not losing out anything on this marriage (she and the children are just as pasul with this marriage as any other marriage)--therefore, based on Rashi's CHiluk, the Mamzeres I think should get a Ksuba. Although Mamzeres is an Issur D'oraisa, Rashi is not saying that has any role in the Din of Shniyos (i.e. that it is an Issur D'rabonon). Rather, he is Mechalek only on the basis of a determination of who most likely instigated the marriage.
Dov Reichman, NY,NY, US
The Gemara in Yevamos (85b, and Rashi there, DH Kansu) explains that wherever either she makes him and his offspring Pasul, or he makes her and her offspring Pasul, the Chachamim did not penalize her with the revocation of her Kesuvah. The reason for this is that they only penalized her where they preferred that the husband be quick to divorce her when they get into an argument (normally, when a husband and wife get into an argument, the Kesuvah prevents him from being quick to divorce her). Here, though, when one of them is making the other Pasul, there anyway will be strife among them and he will divorce her even if she has her Kesuvah.