[kawainu]

Ep 5:

So, the first phone imouto was Tsuruma Konoe (black haired girl). It is unlikely that she would try to mess with the main character again so we have to assume the phone imouto at the beginning of this episode was a different girl. There is no way to be sure if the new phone imouto really is his little sister but I think I know her identity. During this episode there were 2 or 3 calls from this new phone imouto, but there is a clue in the first call (I'm assuming all phone calls during this episode were made by the same imouto). During the first phone call, the imouto referred to herself as "watakushi" instead of the more common "watashi". In anime, "watakushi" is mostly used by wealthy high class characters, and in this specific anime, there is only 1 girl who uses "watakushi": the blonde girl of the student council. That doesn't mean she is the real sister but it means she is probably the bitch behind the phone calls.