Onee-sama wa Hoken'i "Otouto ga Sukisugite, Nakisakebu Hodo Ijimetaku Nacchau no♥"

This spring, the protagonist, Natsume Tsukasa (name customizable), is set to attend a local all-boys school.
Tsukasa, who’s always struggled with communication, feels a mix of anxiety and excitement about starting a new life in a new environment.
But on the day of the entrance ceremony, Tsukasa is practically kidnapped—by his older sister, Natsume Sakurako.
Sakurako, who works as the school nurse at an all-girls school, pulls some strings to enroll Tsukasa there under a false identity. All of this, of course, is for the sake of her beloved little brother…
Forced to go along with it, Tsukasa starts attending the girls’ school but struggles to find his place in the classroom, eventually resorting to spending most of his time in the nurse’s office. Sakurako, who’d planned for this all along, dotes on him—sometimes gently, sometimes with a bullying streak (in a very suggestive way).
Despite his protests, Tsukasa’s body betrays him, growing more and more responsive to his sister’s… attentions.
Before long, Tsukasa catches the attention of his childhood friend and a mysterious classmate. Once his secret is out, he finds himself at the mercy of not just his sister but these two girls as well.
With no time to rest or study, Tsukasa is subjected to daily “experiments” under the guise of “research,” as the girls keep extracting his semen.
And so, in the nurse’s office, their teasing continues…

[Translated from dlsite]