Yevamos, chapter 13, mishnah 8. The mishnah deals with a man who married two women and died, and with the question of when the yibum or chalitzah of one of them exempts her co-wife. The principle that emerges from its words rests on the strength of each woman's kiddushin: whether her betrothal is valid by Torah law or only by rabbinic law.
Two minor orphans:
"Mi shehayah nasui lishtei yesomos ketanos umeis" - a man who married two women, both of them minor orphans who had not yet reached the age of mitzvos. The kiddushin of both is only rabbinic. What, then, is his brother required to do?
The mishnah says: "Biasah o chalitzasah shel achas mehen poteres tzarasah" - whether he performs yibum with one of them or gives her chalitzah, her co-wife is exempted from chalitzah. The reason: both marriages stand on the very same level, both of them rabbinic.
"Vechen shtei chershos" - the same law applies to two deaf-mute women, whose kiddushin is likewise rabbinic. And there is no concern here of a forbidden relationship, since the two women are not relatives of one another.
A minor and a deaf-mute:
When one is a minor, whose kiddushin is rabbinic, and the second is a deaf-mute, then even though both are betrothed by rabbinic law, they are not of one type, and therefore their force is not necessarily equal. Accordingly we learned: "Ein bias achas mehen poteres tzarasah" - the yibum with one of them does not exempt the other, and the second requires chalitzah.
A fully capable woman and a deaf-mute:
One is fully capable - a whole and healthy woman who is not a deaf-mute - and the second is a deaf-mute. The fully capable woman is betrothed by Torah law, whereas the deaf-mute is betrothed by rabbinic law:
"Biasah shel pikachas poteres es hachereshes" - the yibum with the fully capable woman exempts the deaf-mute, because her marriage stands on a lower level, rabbinic only.
"Ve'ein biasah shel cheresches poteres es hapikachas" - the yibum with the deaf-mute does not exempt the fully capable woman from her obligation, and she must be given chalitzah.
An adult and a minor:
The law is the same when one is an adult - grown up, whose kiddushin is by Torah law - and the second is a minor:
"Biasah shel gedolah poteres es haketanah" - the yibum with the adult exempts the minor from her obligation, since it was done on a higher level.
"Ve'ein biasah shel ketanah poteres tzarasah gedolah" - the yibum with the minor does not exempt the adult, because her marriage stands on the level of Torah law, and therefore chalitzah must additionally be given to the adult.
In summary: in this mishnah we learned that everything depends on comparing the levels of kiddushin. Two women whose kiddushin is on one level and of one type - two minor orphans or two deaf-mutes - the yibum or chalitzah of one exempts her co-wife. Two whose kiddushin is rabbinic but who are not of one type - a minor and a deaf-mute - the yibum with one does not exempt the other. And when one is betrothed by Torah law and the second by rabbinic law - a fully capable woman and a deaf-mute, an adult and a minor - the yibum with the one on the higher level exempts the other, whereas the yibum with the one on the lower level does not exempt, and the co-wife requires chalitzah.