Editing Mesechtas Yevamot (section)

==== Shomeret Yavam ====
A shomeret yavam is a woman whose husband died without children, and therefore she is bound to yibum, and she "guards" (from the expression "and his father kept the matter") and awaits yibum.

There is a dispute in the Gemara whether such a woman has the laws of marriage regarding the yavam, since she is bound to him to betroth her and she is forbidden to the entire world. If so, it is considered that the Torah extended the marriage of the deceased brother to the living brother. This dispute is called in the Gemara "Is there zikah (bond) or is there no zikah." If indeed there are marriage laws upon her and "there is zikah," the Sages instituted that she should indeed have all the laws of marriage prohibitions, which are prohibitions regarding relatives.

One of the disputes arising from this question is whether if a shomeret yavam died, is the yavam permitted to marry her mother, or do we say that it is considered as if she is his wife's mother, and she is forbidden to him by the prohibition of a mother-in-law. Rav's opinion is that her mother is permitted, but Shmuel ruled that her mother is permitted, and all the poskim have ruled accordingly.