It seems totally unnecessary to hide from Sakura the whole childhood friend thing. I can understand trying to hide it from classmates, since they see them every day and naturally everyone would know they are not real childhood friends (since they know Kaede is a transfer student). Even if hiding that scheme has led most to assume Kouhei and Kaede are dating or close to it. But what does Sakura matter in this sense? She's just Kouhei's sister's friend. Explaining it to her would prevent her from making mistakes in the future. The same goes for the sister herself and the other friend. Since they are older, they would merely consider it the quirk of younger kids, especially since it's nothing harmful. It's Kaede's obsession, so when explaining it to others, Kouhei could always say it was Kaede's idea he accepted.