פירוש על קידושין 102:5

If one betroths one of his daughters but does not remember which one (wife is not going to be happy about this situation!), we are not concerned that he betrothed one of the adult daughers, because he cannot marry off his adult daughters. But we are concerned about the minors. Any one of them might be the one betrothed. However, this is a case of kiddushin that cannot be followed by intercourse. The man cannot have intercourse with any of the minor sisters lest she is his wife’s sister (meaning he actually married the other one).