IIRC the Duchesses are the half sisters of the Dukes of Taran. Their mothers were never actually married to the Dukes. They were just women brought in to give birth to girls so they didn’t need to be noble they could be commoners.

Firstly, Phillip and his ancestors that became the Taran family doctors NEVER use noble women to be treated (with mugwort) as the Taran bride. It’s already mentioned that Phillip got orphaned girls (virgins, of course) to be treated with mugwort and then one way or another, sent them to be Hugh’s bed partners. It must be the same that happened to the partners of the Dukes (who supposed to produce females that later would be mated with the Dukes to produce male heirs). Because it would be a big problem if it were known that the Taran family did anything strange to noble women that became the duchesses. That’s why no noble women would be chosen to be the Duchess of Taran, ever…. Until Lucia.
It also mentioned (couldn’t remember the exact chapters… Sorry…) that before Hugh, the former Dukes of Taran were indeed never involved in the imperial politics. And the Taran family close themselves up in their castle, never got along with other nobles in the North itself let alone the nobles in the capital or other regions of Xenon.
With the Taran’s scarry reputation, nobody tried to question about the origin of the Duchesses or any personal matters inside the Taran family.
The former Duchesses of Taran (before Lucia) seemed to never appear at all in the society of the North, and even the former Dukes (before Hugh) also rarely went to the capital.
And it’s already explained that the former king (Kwiz, Katherine & Lucia’s father) never bothered the Taran family to maintain good relationship. I think the kings before him also did the same.
That’s why the incest practice inside the Taran family and whatever the Phillip family did with their ‘experiment’ kept going for god knows how long.
It’s like “what happened in the North, stays in the North”… You know 🤷‍♀️