Ore no Imouto ga Saikou no Okazu Datta

Synopsis

My younger sister Miyu caught me fully enjoying a "younger sister" eroge! She's always expressionless and a bit cold, but for some reason, she ends up helping me masturbate!... doing things like cumming on my sister's skin and getting her to touch it! After so much skinship, my body won't respond to anyone but my sister! (Source: France Shoin, translated)