Kiddushin 2:6-7

Kiddushin 2:6

If a man betroths two women with something worth a prutah, or one woman with something worth less than a prutah, they are not betrothed, even if he follows up with engagement presents. This is because he sent them as unconditional gifts, thinking they were already betrothed. The same is true if a minor tries to betroth a woman (i.e., it is ineffective even if he follows up with presents after he reaches the age of majority).

Kiddushin 2:7

If a man simultaneously betroths both a woman and her daughter (which is prohibited) or a woman and her sister (which is also prohibited), they are not betrothed. There was an incident involving five women, two of whom were sisters. A man gathered a basket of figs that belonged to them but it was the sabbatical year (so the produce was free for the taking). He said, “You are all betrothed to me with this basket” and one of the women accepted it on behalf of all of them. In this case, the Sages ruled that the sisters were not betrothed (but the other three women were).