@DizzyDazzle
You really should stop applying modern sensibilities to feudal era societies. First off many political marriages are only marriage in name. Copulation happens o ly to produce an heir and that's as far as it goes. Usually the second marriage or the mistresses are childhood friends or those they have come to care about. So in society where multiple wifes and mistresses are prevalent, it's not considered cheating. Not only is it not cheating, its consided normal and in some cases mandatory. Sickness, pregnancy issues, still births, kidnappings, assassinations, or accidents all can easily rob nobility of their heir. So backup are produced. Granted it leads to many situations of jealousy, abuse and murder, but it is a historical fact that it was and still is a common practice in many countries.
@Fuew12
I would say it would have more to do with who married into whose family, and whose family had more influence before the marriage. Her standing in noble hierarchy might be higher the the dad's due to being from a more prominent family. So threating to tell mommy would be more of a reminder of her family's higher rank, and that they might apply pressure if events are not to their liking. Sadly dad seems like more of a Chuck, and rather tired and worn. So even if he has the higher influence and standing, the step-bitchmonther has likely been raging on him for years like the Karen she is.
OTOH, the way events are leading, I doubt they would have much grounds to stand on. The rumors are true, and the MC now has independent titles, achievements, and connections of his own. So acting antagonistic twords him would only hurt their own interests. *shrug* This is why politics gets so murky.