Yevamos, Chapter 3, Mishnah 4. This mishnah expands the principle we saw earlier - the law of 'achas lechukasah' - and applies it to additional forbidden relations. The mishnah states: "Sheloshah achim, shenayim mehen nesu'im shtei achayos" - three brothers, two of whom are married to two sisters, and likewise the other pairs that the mishnah lists.
The pairs the mishnah lists:
"Shtei achayos" - two of the brothers are married to two sisters.
"Ishah u'vitah" - a woman and her daughter, whom too a man may not marry both of.
"Ishah u'vas bitah" - a woman and her daughter's daughter.
"Ishah u'vas b'nah" - a woman and her son's daughter.
The reason for the law:
All of these women are forbidden relations to one another, and therefore the very same law of 'achas lechukasah' applies to them. Just as with two sisters who fall to yibum together the yavam is forbidden to both of them, since each one of them is the 'achas lechukasah' of the other, so too with these forbidden relations: in the case of a woman and her daughter, the daughter is a forbidden relation with respect to the second yevamah, and we are left with a situation of 'achas lechukasah' that prevents the yibum.
"Harei elu cholatzos velo misyabmos":
Chalitzah is required, since on the level of the derashah the obligation of yibum still exists. But actual yibum cannot be carried out, for it would bring the yavam to his 'achas lechukasah' or to the daughter of the yevamah, and in either case this is a situation of a forbidden relation.
The view of Rabbi Shimon:
"Rabbi Shimon poter" - Rabbi Shimon exempts them, holding that not even chalitzah is needed. He derives from the verse that in any case where two yevamos fall together before a single remaining brother, or before two remaining brothers, and the yavam cannot marry both women, this is not merely the law of 'achas lechukasah' creating a prohibition, but a Torah prohibition to the point that they are entirely exempt from yibum. Where it is impossible to perform yibum with both yevamos, there is no obligation of yibum at all.
If one of them was forbidden to him as a forbidden relation:
As we said earlier, if one of the two women - whether they are sisters, or a mother and her daughter, or a mother and her mother's mother - is forbidden to him in her own right as a forbidden relation, such as when she is his mother in law, the law is "asur bah umutar bachaverta" - he is forbidden to her but permitted to the other: he is forbidden to the forbidden relation, but permitted to her sister, or her daughter, or her daughter's daughter, as the case may be. For that relationship of forbidden closeness pertains to the other person, and since he has no obligation of yibum toward the yevamah who is a forbidden relation to him, he is therefore permitted to her sister.
Issur mitzvah and issur kedushah:
As we learned in the previous mishnayos, if she is not a forbidden relation to him and is not his mother in law, but rather she carries an issur mitzvah - that is, a sheniyah, a rabbinically forbidden relation - or an issur kedushah, such as when he is a kohen and she is a divorcee, the law is "cholatzos velo misyabmos" - they perform chalitzah but do not undergo yibum: yibum is not possible because of the issur mitzvah or the issur kedushah, but the requirement of chalitzah still stands, since on the Torah level the obligation of yibum remains in place.
In summary: this mishnah extended the law of 'achas lechukasah' from two sisters to the other forbidden relations - a woman and her daughter, a woman and her daughter's daughter, and a woman and her son's daughter - and in all of them the law is chalitzah and not yibum, while Rabbi Shimon exempts them even from chalitzah. We also saw the distinction between a forbidden relation, where he is forbidden to one and permitted to the other, and an issur mitzvah or issur kedushah, where they perform chalitzah but do not undergo yibum.