
so if my conclution is rigth that old man saying hue should get daugther in order to preserve taran blood and let that girl marry to that hidden son huh ??? ( but on the other hand lucia is not from taran family so new born daugther will not be pure blood also I don't think there are other women who has a pure taran blood) ╮( ̄▽ ̄)╭

Yes. Damian is not full blood but he is about 75% Taran. The bloodline will only conceive a boy if a Brother and sister conceives. Basically the woman needs to be from taran family and since there’s usually only 1 boy every successor they don’t have any choice but to sleep with their siblings. If they conceive outside the taran blood the child will always be a girl.

And if I recall correctly, the girl is infertile with anyone that's not of the Taran bloodline? I may be misremembering that but I'm like 70% sure that that's why the Taran bloodline would halt without incest.
To further add upon that plant thing tho, they are essentially trying to erase her genetic presence in the babymaking process. Unfortunately, babies cannot be had without menstruation so the Taran family either keeps or selects a young girl and makes her start taking the plant as soon as her first period comes so as to purify her human body and accept only Taran genes. So yeah... They aren't just sister fuckers but child bride snatchers too.

So two things happened. In her first life, her "dream" as she calls it, Lucia was neglected and starving so she began eating a plant that grew around the palace when she was young, before her menstruation hit. After she got married to that horrible Count, she soon realized that she couldn't get pregnant. She didn't know why until she met a certain traveling doctor who asked if she ate a certain herb, after he knew what was wrong with her, he prescribed her a special concoction that put her body back to normal. This gave her her period back and the ability to have kids but she was already getting on in life so she never had them.
In her current life, Lucia who was aware of the other life, began to willingly eat the plant that would stop her period. Duke Taran or a plan of escape weren't even a thought in her mind. All she could think about was that if she had to marry the count again, she'd rather be barren. Fast forward to the time she meets the Duke, she knows for a fact that she can't have children with no period and that he already has an heir in mind. What she doesn't know is that the wandering doctor who she met by chance in her past life is actually the Taran family doctor that the Duke despises. She also doesn't know that in order for the Taran child brides to be pure enough to accept only Taran genetic information during conception, they have to eat the same plant that she was eating for a year which has the side effect of stopping their periods. She especially doesn't know that the doctor really wants the Duke to continue the tradition and have a daughter or that she just so happened to already fulfill the requirements for the special concoction needed to give her back her menstruation and therefore the ability to have the Duke's daughters.
Ohhh whitey and anstasius dear gooood now they'll be couple forever in my head lolbヾ(☆▽☆) most satisfying troll ever <3