https://www.dlsite.com/books/work/=/product_id/BJ173018.html
https://www.dlsite.com/books/work/=/product_id/BJ222540.html
https://www.dlsite.com/books/work/=/product_id/BJ01155778.html (English)
https://coolmic.me/titles/2784 (English)
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We Were Just Pretending to be Lovers, But Now I Can't Stop Moaning... I Can't Stop Myself From Penetrating My Big Boobed Older Female Cousin

"My body's so hot... and I can feel... something leaking...!" I would have never thought I would end up having sex with a boy I'd only thought of as a brother.
When they were kids, there was a nice older girl, Akari, who used to play with Yamato all the time. Now, Akari's back from college.
Yamato had always had a thing for Akari, so he invited her out to a festival in their home town... But hidden away from most others, they suddenly come across a horny couple having sex outside! They knew they shouldn't watch, but couldn't help it. When the man hears them, they have to pretend to be lovers so he doesn't find out that they've been spying on him and his girlfriend!
But as Yamato and Akari get closer, the feel of her boobs and her butt is just too much for Yamato to handle, and he is overcome with lust! Will they end up having sex outside just like the previous couple...?!

Japanese RAW: https://e-hentai.org/g/1708295/31d4f9405f/

Chinese Translation: https://e-hentai.org/g/1718193/1cdc023966/