Commentary for Kiddushin 129:12

Why does the mishnah need another example of the same principle? Why does it need to teach us that if the mother says the daughter was betrothed to this man, the mother is not believed vis a vis the daughter?
The answer is that the rabbis did give the mother the power to betroth the daughter when the father is no longer alive. So we might have thought that the mother is believed and creates a prohibition for her daughter. Therefore, the mishnah teaches that she is not.