Yoshizawa Vanilla

Link: The Doujinshi & Manga Lexicon

Keiichirou Mishima is a 15-year-old guy living with his mom until she suddenly decides to fly to Paris to be with Keiichirou's dad. His mom arranges with his uncle Shirasaki to let him stay at his mansion with his two cousins Marie and Tomari, who he met many years before when he stayed with them for a short while. However, Keiichirou has forgotten nearly everything about back then, including why his cousins apparently now hate him.