The Rustichels are a County (continental european equivalent of an Earldom), and they can easily be a part of High Society, despite the intended" average noble family" image the author wants to portray. So not only the biatch is being a biatch and almost hits a child based on preposterous claims, she also has no backing whatsoever for these loathsome allegations.
Moreover, it's not only a matter of her being despicable and shallow, it's also her utterly disregarding her rank and that of the Rustichel family. She fully deserves that official complaint, and that's an easy way out for her, given that she's only a Viscountess. Lady Rustichel, having higher backing, could completely destroy her social life.