Onee-chan to Asobou

Hiro finds himself spending time at home with an older girl from the neighborhood, who his mother hired as his babysitter on weekdays. He sees her dozing off, and because he was keenly interested in her body, he takes a peek at her panties like a naughty boy. However, the older girl wasn't actually asleep, and she demands a "sincere" apology from him for his unacceptably naughty act... To check if Hiro can ejaculate sperm before puberty, she performs a handjob on him. He gets his first orgasm from her toying around with him. The order to show his "sincerity" just keeps escalating. The older girl then comes clean to him with the words: "The truth is, I'm a perverted older girl who really loves to play around with boys like you, Hiro." Her thick body will make him ejaculate like an adult for the first time. (Source: DLsite, translated)