I believe if we are around sarcasm a lot (especially raised in it), it would take considerable effort to change. It can be like a very bad habit. It is possible to retrain ourselves, if we are willing.
I am not that good at wording statements, however, maybe you could say something in a gentle voice that is telling her whenever she makes sarcastic remarks, it affects her beauty. I would start there and if she says she'd like to break herself from doing it.....that will be great. Difficult to not fall back when she's around the parents. If she doesn't care, then you'll know it's going to be tough.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!
