Why did Sakurako call Akari a "betrayer"?
The answer for this is actually funny: because Akari and Sakurako are both flat chested, and in anime, girls (particularly flat chested ones) sometimes identify other girls as either friends or enemies depending on their breast size. In this situation, Sakurako thinks Akari is a friend. When she mistakes Himawari for Akari, she thinks that Akari grew big boobs and left her alone being flat chested, and she yells "You Traitor!"

If you don't believe me (it is a bit hard to believe) watch some harem anime. For example, the same situation happens in Negima.