https://www.dlsite.com/girls/work/=/product_id/RJ01151542.html

Description:

Ichinose has been assigned to the famous author, Tsubaki Sumihiko, and he is struggling with Sumihiko's difficult personality, but he manages to do his job.

His only solace is a relative, Tsubaki Kanako, who lives with Sumihiko.

One day, Ichinose realizes that he has left his notebook at the Tsubaki mansion. When he returns to find his notebook, a sweet voice from the nearby room shakes him to his core.

As he peeks into the bedroom with its door slightly ajar, he finds Kanako kissing Sumihiko as if she were comforting a child...