Sister having sex

I lived with my sister as a teenager, and i heard her say to her husband she did not want sex as i was next door, and there were i times i would be out, and they could have sex then, a few more moments later I heard the banging of the headboard, groaning, and slapping of flesh.After that i just tried to spy on them and caught them several times at it, and on one occasion, my sister cheated (she thought i was at college)