Shounen ga Otona ni Natta Natsu Episode 4

Ryuuki Kirishima, the protagonist, wonders why his older sister Rinko hasn’t shown him her room in four years. When he finally opens the door, he discovers Rinko’s true nature, leading him to an emotional release amidst the chaos. His feelings intensify to a critical point, and the secret between them will linger into their adulthood.