While I don't like this couple as the characters (weird that I still continue to read this manga), I want to slap that granny principle senseless and even if she understood that she's wrong, I'd still slap her senseless anyway.
also important how she is show this chapter to not be a perfect infalible oracle as some "voice of reason" adults are portrayed, she realized her wording last time was not the best and made the effort to correct posible missunderstandings in her message, even changing a bit her stance on the matter without contradicting it.