Kiddushin, chapter two, mishnah 7. The mishnah opens: "HaMekadesh ishah u'vitah, o ishah va'achosah k'echad - einan mekudashos".
Explanation of the words:
"HaMekadesh ishah u'vitah" - one who attempts to betroth a woman and her daughter together.
"O ishah va'achosah k'echad" - one who attempts to betroth a woman and her sister at one time.
In both of these cases the man is trying to betroth two women who are forbidden relatives to one another, and he cannot be married to both of them. Had he betrothed them one after the other, the second betrothal would not have taken effect; here he betroths them simultaneously, and therefore "einan mekudashos" - the betrothal does not take effect at all. This law is derived from a verse: wherever a man betroths two forbidden relatives together, the betrothal does not take hold even with one of them.
The incident with five women:
The mishnah relates an incident from which the Gemara derives several laws:
"U'maaseh b'chamesh nashim, u'vahen shtei achayos" - among the five women there were two sisters.
"V'likeit echad kalkalah shel te'einim, v'shelahen haysah, v'shel shevi'is haysah" - a man gathered a basket full of figs that belonged to these five women, except that it was the shemittah year, and produce of the seventh year is ownerless and belongs to no one.
"V'amar: harei kulchem mekudashos li b'kalkalah zo" - all of you are betrothed to me with this basket full of figs.
"V'kiblah achas mehen al yedei kulan" - one of the women accepted the basket of figs for the purpose of betrothal on behalf of them all.
"V'amru chachamim: ein ha'achayos mekudashos" - the two sisters are not betrothed, and from the implication of their words we learn that the betrothal did take effect with the other women.
The laws derived from the incident:
One who betroths with an object he stole from the woman - the betrothal is not valid. The betrothal took effect here for the three women only because these were shemittah fruits; had they not been shemittah fruits, the fact that the basket belonged to them would have created a problem, and it was necessary that they be shemittah produce in order for the betrothal to be valid.
Agency between co-wives - even though co-wives are not believed regarding one another, such as to say that their husband died, and likewise in other areas of halachah, this does not detract from the fitness of one to serve as an agent for the other, even though through this very act they are becoming co-wives with one another.
The Gemara's inference:
The Gemara makes an inference from the first law in the mishnah, that one who betroths two forbidden relatives - two sisters, or a mother and her daughter - together, his betrothal is ineffective. The implication is that if he betrothed only one of the two, without identifying which of them, the betrothal does take effect. Let us distinguish between two formulations:
He said, "One of these two women is betrothed to me": one of them is betrothed, but it is impossible to identify which, and therefore both of them require a get, since it is not known which of them is betrothed to him. But as far as marital relations are concerned, he may not live with either of them, since he does not know which of them is the wife and which is the forbidden relative.
He said, "Whichever of you is fit for marital relations is betrothed to me": since in practice neither of them is fit for marital relations - for he cannot live with either of them on account of the doubt - the betrothal does not take effect with either of them, and neither of them requires a get.
The Gemara therefore understands that in the incident in the mishnah, with the five women, two of whom were sisters, the man used this formulation: I hereby betroth with these figs every woman who is fit for marital relations, whom I am permitted to live with. It emerges that the betrothal takes effect only with the three women who are not sisters. But as for the sisters, since we do not know with which of them the betrothal took effect, and although one of them is fit to be betrothed it is not known which, he consequently cannot live with either of them, and therefore the betrothal does not take effect with either of the sisters.
In summary: in this mishnah we learned that one who betroths two forbidden relatives together - his betrothal does not take effect at all; and from the incident with the five women we learned that one who betroths with an object he stole from the woman, his betrothal is not valid, and that a co-wife is fit to serve as an agent for her fellow even though co-wives are not believed regarding one another. We also clarified the distinction between one who betroths "one of you" without specification, whose betrothal takes effect in doubt so that both women require a get, and one who betroths "whichever of you is fit for marital relations," whose betrothal does not take effect at all - and with this the wording of the mishnah, "the sisters are not betrothed," is explained.