פירוש על קידושין 129:7

The man claims he has betrothed the woman, and therefore he is believed with regard to himself. The consequence is that all of the woman’s relatives (daughter, mother, sister etc.—for a full list see Yevamot 4:7) are prohibited to him, because according to him, he is betrothed to their relative. However, the woman denies that this man betrothed her. Hence she is not prohibited to his relatives.