Pet Mimamori Camera ni Ane to Otouto no Sex ga Utsutteta.

My name is Rina, and I have become completely enthralled by the abandoned cat that my older sister Mina and my younger brother Masao picked up, so in order to keep tabs on it while I'm away, I had a pet camera set up without my family knowing. So I accessed the camera and checked the footage from yesterday, starting from the time my classes ended, up until I reached cram school, but what I saw captured on video was... Not only my beloved kitty, but also my older sister and my younger brother having sex... (Source: DLsite, translated)