The Duchess looks like someone who needs major trauma therapy. Forced into a loveless marriage, her only value being as a "baby factory" to produce an heir, having great difficulty and nearly dying in childbirth, and then to have her son cursed by the Emperor so that his son could escape said curse---thereby subjecting her to shame and abuse culturally---even her husband held an open affair and then died while living it up with his lover. The Duke probably had his own trauma history resulting in his inability to treat his political marriage partner who was as much (if not more) of a victim as he was with empathy and love as a fellow sufferer. The Duchess may have had her own problems, but it seems like due to the ongoing trauma history they lacked the means to love each other, even if their love was developed as friendship (agreeing to have a polyamorous relationship because they lacked romantic attraction to one another, but nonetheless requiring one another to produce heirs, love them, and treat them respectfully/with honor so they can grow up to not be as screwed up as they are/were. We saw a good examples of their traumas by their ongoing relationships and their inability to love their son despite what they all as a family went through, thereby making each a persistent victim of multigenerational trauma that was passed down from parent to child and so on until Ibee began to break the cycle of abuse. As a result of their combined inability to effectively love Alejandro, he suffered grave abuses, neglect, and traumas from his curse, from other people and the staff/caregivers, and from his own parents.
This is what multigenerational trauma and abuse looks like. "Fault" isn't necessarily an effective statement to describe it because much of the problems (in terms of knowledge of what they are doing and the help/support to fix it) are outside of their controls. The nobles/royals effectively cannot show "weakness" to their "subjects" and thus are forced to never have the social comfort and support that they require in order to do effective jobs, to perform those duties and thus protect their society as a whole. The inability to "talk about it" or to have people with whom they can converse leaves these matters to fester unattended. Thus one of the greatest evils of "class systems" are their destruction of the communal bonds we need to effectively love and help one another as people.