That`s the Reason why I`m Going to Fuck My Niece / Ore ga Mei Kanojo wo Okasu Wake / 俺が姪[かのじょ]を○す理由[わけ] / 내가 조카를 범하는 이유
Ooki Ryuuji dislikes his brother because he thought he was the one to marry his wife. One day, he stumbles on a love potion he had overlooked. He puts it on his brother's daughter, and this marks him into going down the path of controlling and fucking both his sister-in-law and his niece. His perversion and sadism are voracious by degree.