https://book.dmm.co.jp/product/6227538/b915awnmg03804/ https://www.dlsite.com/books/work/=/product_id/BJ02225291.html If you like this work, please 𝐒𝐮𝐩𝐩𝐨𝐫𝐭 𝐭𝐡𝐞 𝐀𝐮𝐭𝐡𝐨𝐫. 代传,nekomoe自购 机翻勿扰
Chapter 3: よこのうち is a weird Netorare story, tag that accordingly.
Tenko brings her boyfriend Kamata home to meet her whole family to say they are about to start living together, things seem normal as Kamata integrates with Tenko's family... then he suddenly catches Tenko having sex with her own father, and for some bizarre reason they act like it is the most normal thing in the world, it is not some secret affair, her mother knows, her brother also knows as they casually approach them to talk about the dinner arrangement like nothing unusual is happening.
Kamata cannot believe what he saw so he just tucks himself on the bathroom, as he loses track of time and spends hours there; Tenko knocks to see if he is ok, he was there for a long time. Tenko casually talks to Kamata as if nothing happened prior so he wonders if he hallucinated everything? They go to sleep all rounded as family, and now Tenko has sex with her younger brother, he has been looking forward to it, and just like that Kamata once again witnesses his girlfriend having sex with someone else, her own family member again! His mind is utterly lost on how this insanity is happening right before his eyes, but for Tenko it is completely normal, yes, it is something they call "Family Rules" to show love and affection among family.
Tenko and Kamata leave her parent's home earlier than planned, she saw how physically ill Kamata was looking but he never told her why, but she felt they should leave, she is sad she didn't get to tell her parents they are about to start living together; when Kamata finally seems to want start talking Tenko just cuts him and professes how much she wants to be with him forever, to create and maintains lots of "Family Rules" with him in the future... they kiss, Kamata doesn't say anything, he answers Tenko's love and accepts this is her "normal" and is about to become his too, they will visit her parents again soon. THE END
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Edit: The remainder of the tankoubon is fairly tame and traditional though, only that particular story sticks out, Chapter 6 baits with what looks like an affair but it's only a misunderstanding, she was never in a relationship.