Junshin Karen na Amaeta Imouto ni Ochinchin no Osewa o Yaite Morau Seikatsu! ~Onii-chan no Seeshi Pyuppyu shite Aka-chan Tsukutte Kudasai♪!~

Related by marriage, a hentai big brother thinks constantly about his new sister.
She sees him one day while he's masturbating to fantasies of her.
She's too innocent to know what masturbation is and mistakenly believes his penis is hurt.
Without a second thought he asks her to help him make the swelling go down.
It's the start of a taboo life of doting love and discovery!

[From DLSite English]