When two brothers married to two sisters both die childless and it is unknown who died first, what must the surviving yavam do?
Halachah 1 rules that since both sisters are obligated to the same yavam and it is forbidden to marry two sisters, he may not perform yibbum with either.
Question 2
If the wife of the first brother to die is forbidden to the yavam as an ervah, what is the ruling regarding her sister?
Halachah 1 explains that an ervah is never bound by the yibbum obligation, so the non-ervah sister is fully permitted.
Question 3
If a yavam performs chalitzah with two obligated sisters, what is the effect on the deceased brothers' other wives?
Halachah 9 rules that chalitzah with the sisters is 'inferior' and does not free the other wives.
Question 4
A man's wife was married off by her father as a minor, he later divorced her, remarried her while she was still a minor, and then died childless. What is her status regarding the yavam?
Halachah 13 rules she is forbidden because the Scriptural divorce overrides the incomplete Rabbinic remarriage.
Question 5
When the mentally competent brother dies childless and leaves a yevamah who is the sister of the deaf-mute brother's wife, what must the deaf-mute brother do?
Halachah 17 rules that the Scriptural yibbum obligation takes priority over the deaf-mute's Rabbinic marriage, so he must divorce his wife.