Well Done(,) Pervert

Writing lewd thoughts about the adored student council president into a notebook is student council secretary - Kumozawa Koiko. These thoughts were supposed to be a secret, but... StuCo president's younger sister, Nika, stumbles upon the notebook one day?!
It's a start of a comedy where MC becomes a toy for her loved one's younger sister.

(Note: translated from official description)