Not only he is a theoretical genius who tops the school’s written exam scores, he is also an inventor, an elite special forces soldier with plenty of experience killing people, a national secret due to his unique power, a cold and calculating person who has a lovable sister who is a genius in her own right and despite her cool and graceful demeanor; had to restrain herself from outright jumping his bones.

ancient magic

The MC was born with two very specific abilities closer to unique abilities compared to magic, this was unacceptable for the clan he was born in as it is seen as "the" strongest clan in magic. Therefore they performed a surgery to increase his magical abilities. Magic is explained as being a part of a persons brain and as the abilities was taking up almost all of the space they enlarged it to give him average magic abilties at the cost of stripping him of all his emotions.

Except for the love to his sister so to keep a bind on him that could keep him loyal to the clan.

This is one of my most hated parts since his sister uses this (she is in love with him and a total brocon) to her maximum advantage. This becomes a chain to the MC and during the whole story the MC wants to break free of the clan. But because of his sister, the clan uses her by making her the next head and forcing him to marry her. The sister is all too happy with this (my feel and opinion) since it means he will become her husband. Thus making the MC unable to ever break free of the clan that broke him and made him into a weapon.

I have to add though that shat they kept was the MC's love as a brother towards his sister so no idea how that became romantic love, except that MC is a complete pushover when it comes to his sister.

One girl has eyes that allow her to see spritis/magical elements, this is extremly important to a certain clan of magicians so if the clan finds out about it she would be forced to marry into that clan or be directly kidnapped (and most likely forced to carry their children so to pass on the eye's) so it's kept a secret. Though that clans boy and she becomes a couple eventually so perhaps it never mattered?

Still that this is shown as something normal and pretty much goverment approved to increase the amount of talented magicians is pretty dark

Like having a girl from a bloodline that are extremly loyal and does close to an imprinting on a person they truly love so as to always be loyal or love him/her and then while feeling said love for MC an alien parasitic life form comes and absorb that feeling. Thus becoming emotionally attached being 100% loyal and in love? With the MC.

The novel half focus is the relationship of Tatsuya and Miyuki, Tatsuya being born with a specialized magic which makes him be considered useless by the higher ups for his lack of diversity, whereas Miyuki being born with balanced ability, and probably biased towards ice and cold. In his young age, his mother and aunt had him undergo an operation in an attempt to artificially add some magic talent to his brain, they were succesful, but the side effect to this operation was him losing his emotions, and the mother losing the ability to care for him. His mother did set a failsafe measure, making him consider his sister as the only one he can love. The operation was somewhat a success, they did add some ability to use magic outside of his specialized ones, albeit rather weak.

The remaining half of the novel follows Tatsuya and Miyuki outside relationships, with the Yotsuba clan they are both affliated with (aunt is the current head and Miyuki is the next one), Tatsuya being a special soldier with the military, Tatsuya only two abilities (can rewind time, or at least the Eidos, and disintegrate anything), their relationships with their schoolmates and the other big clans, Tatsuya being a genius developer of magic artifacts and devices, and probably Miyuki unending obsession with her brother.

They got married within the Yotsuba Succesion volumes iirc.

Although the LN consists of 32 volumes, I believe the main climax of the story occurs around the 16th to the 18th volume. Even so, the plot doesn't really decline toward the end; it remains quite engaging and enjoyable to read. One of the main things that I disliked throughout the LN is the use of parasites as a plot device. It feels somewhat too foreign for the overall story, and it introduces too many possibilities that end up muddling the world structure and the rules that it has established for itself. Apart from that, there are a few inconsistencies and plot holes (dismissable), but nothing really major except for the last brawl between Tatsuya and Masaki, which didn't fit the characters at all.