Kakure Ecchi

As winter break approaches New Year's, broke college student Sanjou Tomoya hurries home, eager to try out the new porn mag he bought. But the moment he starts getting busy in his room, his stepsister Megumi accidentally walks in. Though she initially freaks out, Megumi — now at that curious age — surprises him by asking to see more. Reluctantly giving in, Tomoya struggles to focus, prompting her to suggest a "show-and-tell" exchange. The next day, their "momentum" carries them straight into a full-blown affair. Now addicted to the thrill, the siblings seize every chance to hook up while dodging their parents — groping under the kotatsu during family time, ambushing each other in the bathroom, sneaking into baths together, or even fooling around right behind their parents doing dishes while pretending to watch TV. Then right before New Year's, relatives arrive. Tomoya's cousin catches them in the act — but instead of exposing them, her price for silence is... more sex. Now he has to satisfy both her and his jealous sister without either finding out. Humping at the shrine visit, using a remote-controlled vibrator under the kotatsu while everyone eats holiday food, making his sister climax on the kotatsu while his cousin sucks him off beneath it... how far will this chaos go?! [Translated from DLsite]