Can A Mother In Law Perform Mi-un?

Dedicated to all of the chushuve yidden who are learning Yevamos and also li-rifuas Tsiporah bas Shaindel and Gittel Tovah bas Yocheved bitoch shear cholei yisrael.

The Rambam [Ishus 7/16] paskens that you can be mekadesh a fetus while in-utero [talk about not knowing the girl before you marry her:-)!]. He adds that afterwards he should marry her again [by giving her father money] so that it is a קידושין שאין בהן דופי - a kiddushin with no aspersions cast upon it, but a good, clear, doubtless kiddushin. What is the difference between a קידושין שאין בהן דופי and a קידושין שיש בהן דופי?

The Rishonim asked on the Rambam: The mishna at the beginning of Yevamos says that a mother-in-law can never perform מיאון because [as the gemara explains on י"ב] having a child makes her a gdola and a gdola can't perform מיאון. According to the Rambam however, we have a scenario where a mother-in-law can perform מיאון. Namely, if a man was mekadesh a fetus of a קטנה. The "mother-in-law" is still a קטנה who can perform מיאון because she hasn't given birth, yet she is already a mother-in-law because of the married fetus she is carrying. How can the Rambam contradict the mishna that says that אין חמותו ממאנת?   

One approach is that if a woman is pregnant with my wife, she is not yet considered a mother-in-law [another nafka minah being that you can't tell mother in law jokes about her yet] so it remains that אין חמותו ממאנת a mother in law cannot perform מיאון, because the moment she actually gives birth she in fact can no longer perform מיאון [and before she gives birth she is not a mother in law].

The famous approach of Rav Chaim Brisker is that indeed there is kiddushin for a fetus but since the fetus is not an "isha" this is a kiddushin-kinyan without "ishus". That explains why afterwards he should be mekadesh again a קידושין שאין בהן דופי. Also, she is not considered a mother-in-law yet because there is no "ishus" with her daughter and the issur of relatives applies only when there is full fledged ishus. Hence we have here a kinyan sans [w/o] ishus. Or, if we wax Brisk - There are two dinim, kinyan and ishus, and the former doesn't depend on the latter.

There is a Ramban in Yevamos [97] who says that goyim are only forbidden to blood relatives but not to relatives through marriage since in order to be forbidden to a relative there must be a proper קיחה [the Torah talks about issurei arayos as יקח]. This can be explained with the principal of Rav Chaim. Goyim can get married but have no concept of "ishus". There is no real אישות of proper קיחה but a מצב of אישות. They are living together, they are married, but there is no true ishus as explicated by the word קיחה. Since there is no ishus, a goy may marry a relative through marriage. That explains how Tamar was allowed to cohabit with her father in law Yehuda. Since they were considered bnei noach, there was no prohibition of "father in law".

This is a broad topic and I hope to revisit it:-).

[Based on an awe-some shiur given in Har Nof by HaGaon Rav Ariav Ozer Shlita]