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): https://e-hentai.org/g/2486148/a129518d48/
Japanese RAW (Chapter 16): https://e-hentai.org/g/2515746/7c4b773647/