Imouto sae Ireba Ii.

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.

(Source: Yen Press)