My Little Sister Won't Let Me Sleep (妹が寝かせてくれない Imōto ga Nekasete Kurenai) is the 5th episode of the Kono Naka ni Hitori, Imouto ga Iru! anime series.
Shougo Mikadono was woken up by a call from his mysterious little sister, who quoted a saying by the principal of the Miryuin Academy in his morning assemblies, revealing that both of them had been attending the same school.Before she end the call, the sister talked about how she was worried about Shougo's staying late habit.
In class, Shougo drew attention to himself when he was daydreaming about marrying Konoe Tsuruma. He confessed his worry to Ikusu Mizutani. Both of them agreed that while Konoe was not the little sister, but she might had inspirated Shougo's sister to use the voice modification function in the prototype phone to contact him. As such, the suspects were narrowed to those whom Shougo had explained his situation to: Rinka Kunitachi, Mana Tendou, and Mei Sagara.
Their conversation was interrupted as Shougo was led to the school's Journalist clubroom. The club's president and editor-in-chief of the Miryuu Academy Times, Araya, was revealed to have an obssession to know everything about Shougo. Shougo then declined to have an interview with Araya.
Every morning in the next few days, Shougo's little sister called him to wake him from sleep. As the result, he could barely get any sleep and became more absent-minded during the day. Just as he was about to hide inside the infirmary for a nap, Mei led him to the Materials Room to do so, in exchange for Shougo to visit her café later on. Mei also revealed that she was only a month older than Shougo.
On Sundam, Shougo and Miyabi Kannagi visited the Lyrical Sisters Café. The day after, Mei talked to Shougo about how she was an orphan picked up by her current parents. Her adopted father was a professor at the University of Massachusetts, while her adopted mother owned the café. When her adopted mother died of illness, and Mei moved to Massachusetts to live with her father. Two years before the series started, she wrote a research paper under her adopted father's guidance, and earned enough money to buy back the café her mother once owned, and restructure it into the current Lyrical Sisters Café.
When Mei was searching for the newspaper that published her research, she accidentially knocked off the stack of books. Shougo had a glimpse of her desk and spotted the same Ryouasu prototype phone. Just as Shougo came to the conclusion that Mei was his little sister, he found out that she was about to sell the café before going back to America.