The climax is not a sexual one—it is linguistic. The rival confronts the trio, demanding she choose. The little sister looks at the rival, then at the protagonist. She steps firmly to the protagonist's side. She utters the line: "My heart and my body belongs to this person. Only he makes me feel complete. He is best."
18+ adult content with optical censoring; it falls under the "netori" category (where the protagonist takes a partner from another). Media Type: Often listed in databases like the Visual Novel Database (VNDB) little sister netori my heart and body belongs best