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If any one of 15 women was married to a man who dies without children, the Mitzvah of Chalitzah and Yibum do not apply to any of the deceased man's wives. (1)
If any of the 15 women died or was divorced, performed Mi'un, or was found to be an Ailonis prior to the death of the brother, the Mitzvah of Chalitzah and Yibum applies to the other wives of the brother. (2)
If the Tzarah of one of the 15 women does Yibum with the other brother, and the other brother dies as well, the Mitzvah of Chalitzah and Yibum do not apply to any of the brother's wives. (3)
If any of the 15 women is a minor at the time of the death of the brother, the Mitzvah of Chalitzah applies, but not the Mitzvah of Yibum.
The prohibition of a mother-in-law is punished with death by burning and the punishment of a daughter-in-law is punished with Sekilah.
A daughter (from an out of wedlock relationship) is listed first in the Beraisa because the prohibition is not explicit in the verse and thus it is cherished by the Tana of our Mishnah.
A BIT MORE
1. These 15 women are: (1) Daughter, (2) Daughter of a daughter (granddaughter), (3) Daughter of a son (granddaughter), (4) Daughter of a wife, (5) Daughter of a wife's son, (6) Daughter of a wife's daughter, (7) Mother-in-law, (8) Mother of a mother-in-law, (9) Mother of a father-in-law, (10) Sister from the mother, (11) Sister of the mother, (12) Sister of a wife, (13) The [former] wife of a brother from the mother, (14) The [former] wife of a man who died before his brother was born, (15) A [former] daughter-in-law.
2. The possibility of Mi'un or Ailonis does not apply to a mother-in-law, a mother of a mother-in-law, or to the mother of a father-in-law.
3. This scenario repeats itself for the Tzarah of the Tzarah.
WHY IS A NIDAH NOT INCLUDED AMONG THE 15 WOMEN
Tosfos asks, why is a Nidah not included among the 15 women to whom the Mitzvah of Chalitzah and Yibum do not apply? Even though she is prohibited only temporarily, the sister of a wife may also be prohibited only temporarily, because once the wife dies the sister is permitted. Tosfos answers that a Nidah is in a different category than the 15 women. The prohibition of Nidah applies not only to the Yavam but to the entire world. Therefore, not only does the Mitzvah of Yibum apply to a Nidah after she becomes Tahor, but even while she is a Nidah -- if the Yavam has relations with her (although it is forbidden) -- the act of Yibum is valid.
REMARRYING WITHOUT YIBUM
A Yevamah who is forbidden to the Yavam with the punishment of Kares is exempt from Yibum and is permitted to remarry without Chalitzah. (The exception is a Yevamah who is a Nidah.) Her Tzaros are also exempt from Chalitzah and Yibum. (Shulchan Aruch EH 173:1)
The distinction of a Nidah from the other women who forbidden with a Chiyuv Kares is that the other women are forbidden only because they are related and they are permitted to everyone else, while a Nidah is forbidden to everyone. Additionally, a Nidah will be permitted upon immersion in the Mikvah.
Tailors who are sewing garments may sew a garment of Sha'atnez in the normal way as long as they do not intend to benefit from the garment, to protect them from rain and sun. The Tzenu'im (people inclined to be stringent) sew on the ground. Merchants may sell garments in the normal way as long as they do not intend to benefit from the garments to protect them from the rain and son. The Tzenu'im drape the garments on a pole. Some opinions even allow one to wear a garment without intention to benefit from it (e.g., wearing a garment for the purpose of avoiding custom duties, or wearing it for the purpose of checking the size). (Shulchan Aruch YD 301:6)
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