
Also spoilers but in the form of a question...........................................................................................................................................................................................
Does he really make his children marry each other because that is just ew

It's a tradition of their family to keep the bloodline strong. The male heirs are forced to have sex and impregnate their sister or woman of the same bloodline. That's why the Duke is so disgusted with the tradition.
The son damien is also a fruit of such a disgusting practice. His father is the late Duke/ twin brother of Hue, Hugo.

Spoiler
So if you read to the end of the novel the author give this feeling of making a new story for the son "Damien" where he will have his own love story and his love interest is a cross dressing girl "gender bender" so for sure no incest to be expected!!
BUT until now I couldn't find his story and that's killing me (〜 ̄△ ̄)〜

Basically, they want to keep the bloodline strong for generation after generation. Hue and hugo are twins who were born from that sick practice. The father of those two only needed one heir, so he wanted to choose one and kill the other.
Damien is not really the son of Hue, he is his nephew. His real father is Hugo, the other twin who killed himself for Hue.

Oh umm spoilers
The Previous duke married his sister and had hue and Hugo. In order to have boys you have to marry your blood relative. I think it was the first time twins happened tho. In order to have girls the women has to be purified like Lucia unknowingly did and then cured.
I’m pretty sure Hugo is the brother and hue is the Male lead. The brother had fallen in love with their sister. Neither of them knew they were related to each other. The brother didn’t even know that she was pregnant when he died I think their father had abandoned her when he found her to be of poor health

Their father had a daughter not from his wife who died with him and he has thrown her away because he thought she was going to die. That daughter fall in love with Hogo "the one who killed his father" without anyone knowing that they were siblings and unfortunately he died before finding out she was pregnant
And to make it worse she died giveing birth to Damien...
The only one who knew about all of that was the doctor Philip

Almost there but a little different. Also, HUGE SPOILER BECAUSE THIS IS BASICALLY ONE OF THE MAJOR PLOTS IN LUCIA. The previous Duke first married his half sister and had twins, who are Hugo (the dead one) and Hue (Lucia’s Hugo). Along side that, just like the Taran tradition to keep the bloodline going, a ‘prepared’ woman for the previous Duke gave birth to their half sister (Damian’s mother). But their half sister was sickly so the previous Duke abandoned her, so Philip the bastard raised her in secret just in case. And that’s how Damian’s parents met and fell in love. The previous Duke married another ‘non-Taran’ woman to give birth to his heir’s (in this case, Lucia’s Hugo) future bride. The Duke and the newest Duchess were the people that Hugo the dead one killed before killing himself. XD

Yes the taran family did that, but then when the male lead twin brother died, hugo decided not to take any woman to be his wife (because Damian already born), but then lucia appeared (then the crazy doctor was so happy because lucia can bear a baby girl so she can married to Damian, ) and actually, hugo just fill the blank of the head of Taran family until Damian is a proper adult, that's the agreement.
Spoiler
That man is obviously Philip. And the lady doctor is someone like an apprentice of his. The tonics that she will be drinking is actually the same thing given to Lucia/Vivian in her "supposed to be future". The herbal drinks she will be drinking reverses her infertility and without knowing this, the Duke got the duchess to become pregnant... Well this will happen way way way way later in the story... I read this from the novel.