Nukitashi

After your parents' death, your older siblings, including your oldest brother Junnsuke Tachibana and your older sister Asane Tachibana, moved to the island of Seiran to lead a decent and normal life. However, there was an absurd law on the island that punished people who didn't have regular sexual intercourse – this is called the "Pervert Law." Anyone who defies or avoids this law is considered a criminal and can be punished. Your siblings, like you, are still virgins, and your oldest brother wants to protect that at all costs. There's also a school on the island that you all have to attend together. You are in the same class as your sister Asana, and your brother is in a higher grade. Asana is gay and lesbian, but once hinted that she might have sex with you if she gets the chance. Normal classes were normal at school, but every half hour after class ends, there's a legal half-hour during which every student is required to have sex, whether with a male or female classmate, in a threesome, or with a partner. The main thing is just sex. It's also possible to get fucked by strangers on the island, and it's completely legal, unless you're pregnant or underage, which on the island is up to 12 years old. After that, you're bound by the law.