Imouto (ni) Koukan (Sareru Ani) no Kai

Mizuho lived a wealthy life with his mother and younger sister, Honoka, thanks to the inheritance of his father, a businessman who died when Mizuho was a child.
However, since his mother died of illness, his sister has discovered Mizuho's weakness and Mizuho finds herself at her mercy.
One day, Honoka orders Mizuho to transfer to Seirakusho Girls' Academy, the school she attends.

Mizuho transferred as a girl.
Mizuho tried her best to fit in with her class, but one day she was called to the student council room, where Honoka and the student council president, Erin, were waiting for her.
Erin was planning to open a brother-pet salon at the academy, and she wanted Mizuho, ​​who is cuter than the average girl, to be the salon's centerpiece.
And from that day on, Mizuho's training began...