Yandere na Onee-chan ni Aishitsukusareru Kankin Seikatsu

I go to the same school as my ideal sister Airi and live a happy life. My recent problem is that I still feel attracted to Airi as a woman, even though I still take baths with her. One morning, Airi finally crawled into the futon. After being forced into a relationship with her older sister who can't resist her love for her younger brother, he realizes Airi's abnormal obsession, but he can't resist her pleasurable service...!?

(Source: Puchira Bunko, translated)