Let's Pretend Last Night Never Happened

"Let's keep this between you and I. We'll play til sunrise." With her parents gone, all the support Chiriko had in life came from her older sister. Soon, the two of them will start living together with her sister's husband's family! Chiriko is fully supportive of her sister's happiness, so going out and hooking up anywhere near the house is out of the question. The night before deciding to lead such a peaceful and quiet life, Chiriko runs into a guy who's totally her type. Their chemistry was just right, so they found themselves together getting down all night. The next day, the same guy shows up at the house Chiriko and her sister are moving into. Turns out he's son of Chiriko's sister's husband?!

(Source: MangaPlaza)