The doujin starts out with the parents leaving for the night and the kids decide to use the night to do some things they aren't allowed to do. I remember that they buy some candy to eat and they talk about how strict the parents are. They also buy some alcohol and drink it, getting drunk in the process. The sister brings up that she snuck a porno video from her friends house and they watch it, despite the brother's protest. While watching the porno, the sister wants the "pretend/reenact" what they are doing and things escalate from there.
I swear I remember this doujin being on Fakku, but I couldn't find it find it using any tags or keywords I can think of. Any help is appreciated :D