Are we supposed to sympathise with this woman, while condemn Kylo's mother?
Both were cheated on, and both seem to hold considerable contempt for their bastard children - given that the marchioness's final straw was the child being adopted, she obviously doesn't think kindly of him.
While the women shouldn't take their anger out at the innocent children, the fathers are ultimately at fault here, and are the real villains in these scenarios.