A Sister’s All You Need.

Itsuki Hashima is a novelist who’s hopelessly enamored with the idea of little sisters and is constantly surrounded by colorful characters. A world class genius and love-guru who’s beauty almost seems a waste on her. A girl who’s constantly troubled by her friendships, love interests, and can’t even find refuge in her dreams. A ridiculously talented illustrator. Each of them have as many problems and worries as the next and they never have a dull day together as they play games, travel, and work together. From the same author of the famous I Don’t Have Many Friends, Yomi Hirasaka! *Source: Yen Press, Vol. 1*

The spinoff manga focuses on Nayuta Kani, who falls in love with the novelist Itsuki Hashima through her literature. Since Itsuki likes little sisters she decides that she needs to become a little sister for him.

Ayu Shirakawa left the countryside and set off for Tokyo. There, her cousin Miyako Shirakawa introduced her to the popular author, Hikari Kairou, and it was decided that she would work for her as both her caretaker, and as a warden. This is just the ordinary love story about how a lonely person became not alone.