In Taishou series, there are five siblings. But the mc in there adapted the surname of the heroine and became a Tachibana after marriage, while the other 2 became a Manabe(?) through adoption. The Older brother died. So the only one left is the crazy elder sister. And being Kotori, the idol singer is still alive, the gentle grand parent is the terror father of five? I can't say that maybe they are in differwnt universe because of Kotori.