1,506. Consecrating Two Related Women
Hilchos Ishus 9:3
Let’s say that a man tells two sisters, “One of you is consecrated to me through this object” and he gives them each a prutah, or one woman accepts it on behalf her sister. In such a case, he must give each of them a get and he is not permitted to have marital relations with either of them. This is because the kiddushin is valid even though he may not have relations with either of them. The same is true if a man tells a father that one of his (unspecified) daughters is consecrated to him and the father accepts the kiddushin from him.
Hilchos Ishus 9:4
Let’s say that a man appoints an agent to consecrate a given woman, which the agent does, but the man himself consecrates that woman’s mother, daughter or sister and we don’t know who was consecrated first. The man must give each of them a get and he is not permitted to have marital relations with either of them. Similarly, let’s say that a woman appoints an agent to accept kiddushin for her, which he does, but she herself accepts kiddushin from another man and we don’t know which man consecrated her first. In this case, each man must give her a get, or one may divorce her and the other may marry her.