פירוש על קידושין 102:10

Again, we encounter a case where a man does not remember which of his daughters he betrothed. R. Meir says we must consider three of the four girls doubtfully betrothed. Only the youngest girl of the younger set is not betrothed. But the husband cannot have intercourse with any of the doubtfully betrothed girls since any one of them may be the sister of his betrothed wife. This seems again to indicate that kiddushin that cannot be followed by intercourse are kiddushin.