Sister Breeder

Omiya Rena, a student council president, and her younger brother, Shota, who has had a obsessive "sister complex" since childhood. With their parents away for a year, Shota's inappropriate advances and harassment escalate from childish pranks to serious sexual coercion. Despite knowing their relationship is wrong, Rena finds herself unable to resist or stop him, ultimately succumbing to his desires and agreeing to become his "sex slave." The synopsis concludes with their younger sister, Aina, returning home unexpectedly, potentially discovering their secret incestuous relationship.

(Source: Hentaimama)