@aodpds711 most likely the cover story of their parents since their age is close to each other, it makes more sense to made them "twins" rather than ordinary brother and sister.
Is it possible that Dad's best friend died the day Kouki was born, which was also the exact day Nao's parents had her, and their shared birthday was a good foundation for the cover story? Because otherwise Kouki's going to have to look into his true birthday.
So how much will the "twins" identity be disassembled?
Is it me or the art looks very similar to Yaki Tomato and the girl looks exactly like his art in 'Confessing To My Childhood Friend Who's Worried She's Plain' ?