https://www.dlsite.com/books/work/=/product_id/BJ01333003.html
https://coolmic.me/titles/2782 (English)
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Let's Learn About Sex: Your Cousin Will Take Your Virginity and Teach You How to Come

Yui Nanjo has problems with studying, so she asked her cousin Sou for tutoring. Since her grades aren't getting any better, she admits that she can't focus as she's recently gotten a boyfriend. Sou gives her condition - he will let her see her boyfriend if she passes his tests. But if she fails... Sou will give her love advice.
Yui ends up failing, which leads to Sou embracing her from behind, his hands brushing against her breasts.
"There's no point if it's not like the real thing," he claims, as his fingers rub her through her underwear. Things escalate to the point of no return as he's aiming to teach her how to cum!

Japanese RAW (Chapters 01-15): /g/2486148/a129518d48/
Japanese RAW (Chapter 16): /g/2515746/7c4b773647/

So, did she break up with the bf or what?

Base +6

Man! Talk about creative censorship...just as bad as traditional censorship.

Base +15, uytgjhgjvbvnbvnbvnb +6, venfare +10

woooowwww~

Base +6

Gahahaha... it is kind of a reverse anti NTR or something? Sou thinks he steals he'd from her boyfriend and that the boyfriend is a daft idiot... but in reality the boyfriend is a SOB. He immidiately sees that she is having sex with her cousin, takes screencap (he doesn't know it is her cousin back then.) He doesn't get all that mad because to start with he just confessed so he could fuck a random girl.

Later he pretends that his mom has passed away... Fucks her and then tells her that he knew all along. He then proceed to fuck her roughly while laughing, finding the whole thing hilarious because he always just saw her as an easy cheap fuck.

Meanwhile Sou, while having been underhanded trying to steal her from the boyfriend actually does love/obsess with her.

So the cousin Sou here is actually the one that got NTR'D.

Base +7