Sex. Lots and lots of sex. The story reason for the sex is that the MC, Kizuna, needs to ejaculate into the girl's wombs so they can power-up etc.
While there's a lot of sex in the LN, the main focus is on Kizuna (17 years old) and his biological older sister Reiri (24 years old). They have tons of sex, and when I say tons I mean
tons. Yes, I'm talking about full-on incest.
Kizuna and Reiri had always been sexually attracted to each other ever since they were younger. Reiri used to touch Kizuna when they were in the bath and Kizuna groped her breasts when they slept together when they were younger (I think Kizuna was 9 while Reiri was 16). They both enjoyed these things. This all culminated later in the story when they needed to perform Climax Hybrid together to provide a power-up, which means they needed to do perverted stuff together. To both of their surprise, they were extremely excited at seeing each other's naked bodies and really enjoyed doing it. Reiri was extremely happy that Kizuna could get so excited seeing her naked body (which is described as the perfect sexy body), and Kizuna likewise. After this, they started to do Ecstascy Hybrid (sex) a lot more. They eventually entered a secret relationship as lovers as neither of them could repress their desire or love for one another as a man and woman anymore, and effectively stopped being siblings in their hearts. At a later point in the story Reiri even ends a conference of country leaders early and has her brother brought to her simply so they can fuck over and over. At this point they do it for hours on end, over and over, until deep into the nights. Fun tidbit; their mother jokingly commented that because they fuck so much Reiri is the girl most likely to get pregnant first out of all the girls in Kizuna's harem. This is because they always do it bareback and Kizuna always ejaculates inside her. Basically, they're absolutely crazy about one another.
There are other girls in the harem that Kizuna is in love with and fucks too, of course. But not nearly as much as he fucks with Reiri. Reiri is actually very possessive of Kizuna. She'll allow the girls the occasional night with Kizuna, but then she takes him back in order to monopolize him for herself. She even abuses her authority at times to monopolize Kizuna. Such as taking a whole week off so she could spend that entire week off getting fucked by Kizuna. The other girls in the harem obviously don't like how she monopolizes him, and is cause for some comical moments.
Here's an illustration of one of Kizuna and Reiri's sex sessions that goes on for hours (WARNING, NSFW):
So yeah, if you like LNs with tons of fucking, and wincest, then you should try to find somewhere to read this LN. Because it has lots of fucking, and
tons of wincest.